[BUG]
Test case btrfs/027 would crash with subpage (64K page size, 4K
sectorsize) with the following dying messages:

  debug: map_length=16384 length=65536 type=metadata|raid6(0x104)
  assertion failed: map_length >= length, in fs/btrfs/volumes.c:8093
  ------------[ cut here ]------------
  kernel BUG at fs/btrfs/messages.c:259!
  Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
  Call trace:
   btrfs_assertfail+0x28/0x2c [btrfs]
   btrfs_map_repair_block+0x150/0x2b8 [btrfs]
   btrfs_repair_io_failure+0xd4/0x31c [btrfs]
   btrfs_read_extent_buffer+0x150/0x16c [btrfs]
   read_tree_block+0x38/0xbc [btrfs]
   read_tree_root_path+0xfc/0x1bc [btrfs]
   btrfs_get_root_ref.part.0+0xd4/0x3a8 [btrfs]
   open_ctree+0xa30/0x172c [btrfs]
   btrfs_mount_root+0x3c4/0x4a4 [btrfs]
   legacy_get_tree+0x30/0x60
   vfs_get_tree+0x28/0xec
   vfs_kern_mount.part.0+0x90/0xd4
   vfs_kern_mount+0x14/0x28
   btrfs_mount+0x114/0x418 [btrfs]
   legacy_get_tree+0x30/0x60
   vfs_get_tree+0x28/0xec
   path_mount+0x3e0/0xb64
   __arm64_sys_mount+0x200/0x2d8
   invoke_syscall+0x48/0x114
   el0_svc_common.constprop.0+0x60/0x11c
   do_el0_svc+0x38/0x98
   el0_svc+0x40/0xa8
   el0t_64_sync_handler+0xf4/0x120
   el0t_64_sync+0x190/0x194
  Code: aa0403e2 b0fff060 91010000 959c2024 (d4210000)

[CAUSE]
In btrfs/027 we test RAID6 with missing devices, in this particular
case, we're repairing a metadata at the end of a data stripe.

But at btrfs_repair_io_failure(), we always pass a full PAGE for repair,
and for subpage case this can cross stripe boundary and lead to the
above BUG_ON().

This metadata repair code is always there, since the introduction of
subpage support, but this can trigger BUG_ON() after the bio split
ability at btrfs_map_bio().

[FIX]
Instead of passing the old PAGE_SIZE, we calculate the correct length
based on the eb size and page size for both regular and subpage cases.

CC: stable@vger.kernel.org # 6.3+
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>