[BUG]
If we remove the subpage limitation in add_ra_bio_pages(), then read a
compressed extent which has part of its range in next page, like the
following inode layout:

	0	32K	64K	96K	128K
	|<--------------|-------------->|

Btrfs will trigger ASSERT() in endio function:

  assertion failed: atomic_read(&subpage->readers) >= nbits
  ------------[ cut here ]------------
  kernel BUG at fs/btrfs/ctree.h:3431!
  Internal error: Oops - BUG: 0 [#1] SMP
  Workqueue: btrfs-endio btrfs_work_helper [btrfs]
  Call trace:
   assertfail.constprop.0+0x28/0x2c [btrfs]
   btrfs_subpage_end_reader+0x148/0x14c [btrfs]
   end_page_read+0x8c/0x100 [btrfs]
   end_bio_extent_readpage+0x320/0x6b0 [btrfs]
   bio_endio+0x15c/0x1dc
   end_workqueue_fn+0x44/0x64 [btrfs]
   btrfs_work_helper+0x74/0x250 [btrfs]
   process_one_work+0x1d4/0x47c
   worker_thread+0x180/0x400
   kthread+0x11c/0x120
   ret_from_fork+0x10/0x30
  ---[ end trace c8b7b552d3bb408c ]---

[CAUSE]
When we read the page range [0, 64K), we find it's a compressed extent,
and we will try to add extra pages in add_ra_bio_pages() to avoid
reading the same compressed extent.

But when we add such page into the read bio, it doesn't follow the
behavior of btrfs_do_readpage() to properly set subpage::readers.

This means, for page [64K, 128K), its subpage::readers is still 0.

And when endio is executed on both pages, since page [64K, 128K) has 0
subpage::readers, it triggers above ASSERT()

[FIX]
Function add_ra_bio_pages() is far from subpage compatible, it always
assume PAGE_SIZE == sectorsize, thus when it skip to next range it
always just skip PAGE_SIZE.

Make it subpage compatible by:

- Skip to next page properly when needed
  If we find there is already a page cache, we need to skip to next page.
  For that case, we shouldn't just skip PAGE_SIZE bytes, but use
  @pg_index to calculate the next bytenr and continue.

- Only add the page range covered by current extent map
  We need to calculate which range is covered by current extent map and
  only add that part into the read bio.

- Update subpage::readers before submitting the bio

- Use proper cursor other than confusing @last_offset

- Calculate the missed threshold based on sector size
  It's no longer using missed pages, as for 64K page size, we have at
  most 3 pages to skip. (If aligned only 2 pages)

- Add ASSERT() to make sure our bytenr is always aligned

- Add comment for the function
  Add a special note for subpage case, as the function won't really
  work well for subpage cases.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>