The recent changes to make bio cloning faster (added in the 4.13 merge
window) by using the bio_clone_fast() API introduced a regression on
raid5/6 modes, because cloned bios have an invalid bi_vcnt field
(therefore it can not be used) and the raid5/6 code uses the
bio_for_each_segment_all() API to iterate the segments of a bio, and this
API uses a bio's bi_vcnt field.

The issue is very simple to trigger by doing for example a direct IO write
against a raid5 or raid6 filesystem and then attempting to read what we
wrote before:

  $ mkfs.btrfs -m raid5 -d raid5 -f /dev/sdc /dev/sdd /dev/sde /dev/sdf
  $ mount /dev/sdc /mnt
  $ xfs_io -f -d -c "pwrite -S 0xab 0 1M" /mnt/foobar
  $ od -t x1 /mnt/foobar
  od: /mnt/foobar: read error: Input/output error

For that example, the following is also reported in dmesg/syslog:

  [18274.985557] btrfs_print_data_csum_error: 18 callbacks suppressed
  [18274.995277] BTRFS warning (device sdf): csum failed root 5 ino 257 off 0 csum 0x98f94189 expected csum 0x94374193 mirror 1
  [18274.997205] BTRFS warning (device sdf): csum failed root 5 ino 257 off 4096 csum 0x98f94189 expected csum 0x94374193 mirror 1
  [18275.025221] BTRFS warning (device sdf): csum failed root 5 ino 257 off 8192 csum 0x98f94189 expected csum 0x94374193 mirror 1
  [18275.047422] BTRFS warning (device sdf): csum failed root 5 ino 257 off 12288 csum 0x98f94189 expected csum 0x94374193 mirror 1
  [18275.054818] BTRFS warning (device sdf): csum failed root 5 ino 257 off 4096 csum 0x98f94189 expected csum 0x94374193 mirror 1
  [18275.054834] BTRFS warning (device sdf): csum failed root 5 ino 257 off 8192 csum 0x98f94189 expected csum 0x94374193 mirror 1
  [18275.054943] BTRFS warning (device sdf): csum failed root 5 ino 257 off 8192 csum 0x98f94189 expected csum 0x94374193 mirror 2
  [18275.055207] BTRFS warning (device sdf): csum failed root 5 ino 257 off 8192 csum 0x98f94189 expected csum 0x94374193 mirror 3
  [18275.055571] BTRFS warning (device sdf): csum failed root 5 ino 257 off 0 csum 0x98f94189 expected csum 0x94374193 mirror 1
  [18275.062171] BTRFS warning (device sdf): csum failed root 5 ino 257 off 12288 csum 0x98f94189 expected csum 0x94374193 mirror 1

A scrub will also fail correcting bad copies, mentioning the following in
dmesg/syslog:

  [18276.128696] scrub_handle_errored_block: 498 callbacks suppressed
  [18276.129617] BTRFS warning (device sdf): checksum error at logical 2186346496 on dev /dev/sde, sector 2116608, root 5, inode 257, offset 65536, length 4096, links $
  [18276.149235] btrfs_dev_stat_print_on_error: 498 callbacks suppressed
  [18276.157897] BTRFS error (device sdf): bdev /dev/sde errs: wr 0, rd 0, flush 0, corrupt 1, gen 0
  [18276.206059] BTRFS warning (device sdf): checksum error at logical 2186477568 on dev /dev/sdd, sector 2116736, root 5, inode 257, offset 196608, length 4096, links$
  [18276.206059] BTRFS error (device sdf): bdev /dev/sdd errs: wr 0, rd 0, flush 0, corrupt 1, gen 0
  [18276.306552] BTRFS warning (device sdf): checksum error at logical 2186543104 on dev /dev/sdd, sector 2116864, root 5, inode 257, offset 262144, length 4096, links$
  [18276.319152] BTRFS error (device sdf): bdev /dev/sdd errs: wr 0, rd 0, flush 0, corrupt 2, gen 0
  [18276.394316] BTRFS warning (device sdf): checksum error at logical 2186739712 on dev /dev/sdf, sector 2116992, root 5, inode 257, offset 458752, length 4096, links$
  [18276.396348] BTRFS error (device sdf): bdev /dev/sdf errs: wr 0, rd 0, flush 0, corrupt 1, gen 0
  [18276.434127] BTRFS warning (device sdf): checksum error at logical 2186870784 on dev /dev/sde, sector 2117120, root 5, inode 257, offset 589824, length 4096, links$
  [18276.434127] BTRFS error (device sdf): bdev /dev/sde errs: wr 0, rd 0, flush 0, corrupt 2, gen 0
  [18276.500504] BTRFS error (device sdf): unable to fixup (regular) error at logical 2186477568 on dev /dev/sdd
  [18276.538400] BTRFS warning (device sdf): checksum error at logical 2186481664 on dev /dev/sdd, sector 2116744, root 5, inode 257, offset 200704, length 4096, links$
  [18276.540452] BTRFS error (device sdf): bdev /dev/sdd errs: wr 0, rd 0, flush 0, corrupt 3, gen 0
  [18276.542012] BTRFS error (device sdf): unable to fixup (regular) error at logical 2186481664 on dev /dev/sdd
  [18276.585030] BTRFS error (device sdf): unable to fixup (regular) error at logical 2186346496 on dev /dev/sde
  [18276.598306] BTRFS warning (device sdf): checksum error at logical 2186412032 on dev /dev/sde, sector 2116736, root 5, inode 257, offset 131072, length 4096, links$
  [18276.598310] BTRFS error (device sdf): bdev /dev/sde errs: wr 0, rd 0, flush 0, corrupt 3, gen 0
  [18276.598582] BTRFS error (device sdf): unable to fixup (regular) error at logical 2186350592 on dev /dev/sde
  [18276.603455] BTRFS error (device sdf): bdev /dev/sde errs: wr 0, rd 0, flush 0, corrupt 4, gen 0
  [18276.638362] BTRFS warning (device sdf): checksum error at logical 2186354688 on dev /dev/sde, sector 2116624, root 5, inode 257, offset 73728, length 4096, links $
  [18276.640445] BTRFS error (device sdf): bdev /dev/sde errs: wr 0, rd 0, flush 0, corrupt 5, gen 0
  [18276.645942] BTRFS error (device sdf): unable to fixup (regular) error at logical 2186354688 on dev /dev/sde
  [18276.657204] BTRFS error (device sdf): unable to fixup (regular) error at logical 2186412032 on dev /dev/sde
  [18276.660563] BTRFS warning (device sdf): checksum error at logical 2186416128 on dev /dev/sde, sector 2116744, root 5, inode 257, offset 135168, length 4096, links$
  [18276.664609] BTRFS error (device sdf): bdev /dev/sde errs: wr 0, rd 0, flush 0, corrupt 6, gen 0
  [18276.664609] BTRFS error (device sdf): unable to fixup (regular) error at logical 2186358784 on dev /dev/sde

So fix this by using the bio_for_each_segment() API and setting before
the bio's bi_iter field to the value of the corresponding btrfs bio
container's saved iterator if we are processing a cloned bio in the
raid5/6 code (the same code processes both cloned and non-cloned bios).

This incorrect iteration of cloned bios was also causing some occasional
BUG_ONs when running fstest btrfs/064, which have a trace like the
following:

  [ 6674.416156] ------------[ cut here ]------------
  [ 6674.416157] kernel BUG at fs/btrfs/raid56.c:1897!
  [ 6674.416159] invalid opcode: 0000 [#1] PREEMPT SMP
  [ 6674.416160] Modules linked in: dm_flakey dm_mod dax ppdev tpm_tis parport_pc tpm_tis_core evdev tpm psmouse sg i2c_piix4 pcspkr parport i2c_core serio_raw button s
  [ 6674.416184] CPU: 3 PID: 19236 Comm: kworker/u32:10 Not tainted 4.12.0-rc6-btrfs-next-44+ #1
  [ 6674.416185] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014
  [ 6674.416210] Workqueue: btrfs-endio btrfs_endio_helper [btrfs]
  [ 6674.416211] task: ffff880147f6c740 task.stack: ffffc90001fb8000
  [ 6674.416229] RIP: 0010:__raid_recover_end_io+0x1ac/0x370 [btrfs]
  [ 6674.416230] RSP: 0018:ffffc90001fbbb90 EFLAGS: 00010217
  [ 6674.416231] RAX: ffff8801ff4b4f00 RBX: 0000000000000002 RCX: 0000000000000001
  [ 6674.416232] RDX: ffff880099b045d8 RSI: ffffffff81a5f6e0 RDI: 0000000000000004
  [ 6674.416232] RBP: ffffc90001fbbbc8 R08: 0000000000000001 R09: 0000000000000001
  [ 6674.416233] R10: ffffc90001fbbac8 R11: 0000000000001000 R12: 0000000000000002
  [ 6674.416234] R13: ffff880099b045c0 R14: 0000000000000004 R15: ffff88012bff2000
  [ 6674.416235] FS:  0000000000000000(0000) GS:ffff88023f2c0000(0000) knlGS:0000000000000000
  [ 6674.416235] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [ 6674.416236] CR2: 00007f28cf282000 CR3: 00000001000c6000 CR4: 00000000000006e0
  [ 6674.416239] Call Trace:
  [ 6674.416259]  __raid56_parity_recover+0xfc/0x16e [btrfs]
  [ 6674.416276]  raid56_parity_recover+0x157/0x16b [btrfs]
  [ 6674.416293]  btrfs_map_bio+0xe0/0x259 [btrfs]
  [ 6674.416310]  btrfs_submit_bio_hook+0xbf/0x147 [btrfs]
  [ 6674.416327]  end_bio_extent_readpage+0x27b/0x4a0 [btrfs]
  [ 6674.416331]  bio_endio+0x17d/0x1b3
  [ 6674.416346]  end_workqueue_fn+0x3c/0x3f [btrfs]
  [ 6674.416362]  btrfs_scrubparity_helper+0x1aa/0x3b8 [btrfs]
  [ 6674.416379]  btrfs_endio_helper+0xe/0x10 [btrfs]
  [ 6674.416381]  process_one_work+0x276/0x4b6
  [ 6674.416384]  worker_thread+0x1ac/0x266
  [ 6674.416386]  ? rescuer_thread+0x278/0x278
  [ 6674.416387]  kthread+0x106/0x10e
  [ 6674.416389]  ? __list_del_entry+0x22/0x22
  [ 6674.416391]  ret_from_fork+0x27/0x40
  [ 6674.416395] Code: 44 89 e2 be 00 10 00 00 ff 15 b0 ab ef ff eb 72 4d 89 e8 89 d9 44 89 e2 be 00 10 00 00 ff 15 a3 ab ef ff eb 5d 41 83 fc ff 74 02 <0f> 0b 49 63 97
  [ 6674.416432] RIP: __raid_recover_end_io+0x1ac/0x370 [btrfs] RSP: ffffc90001fbbb90
  [ 6674.416434] ---[ end trace 74d56ebe7489dd6a ]---
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>