btrfs: shrink the size of btrfs_bio

btrfs: shrink the size of btrfs_bio

This is done by:

- Shrink the size of btrfs_bio::mirror_num
  From 32 bits unsigned int to u16.

  Normally btrfs mirror number is either 0 (all profiles), 1 (all
  profiles), 2 (DUP/RAID1/RAID10/RAID5), 3 (RAID1C3) or 4 (RAID1C4).

  But for RAID6 the mirror number can go as large as the number of
  devices of that chunk.

  Currently the limit for number of devices for a data chunk is
  BTRFS_MAX_DEVS(), which is around 500 for the default 16K nodesize.
  And if going the max 64K nodesize, we can have a little over 2000
  devices for a chunk.

  Although I'd argue it's way overkilled, we don't reject such cases yet
  thus u8 is not going to cut it, and have to use u16 (max out at 64K).

- Use bit fields for boolean members
  Although it's not always safe for racy call sites, those members are
  safe.

  * csum_search_commit_root
  * is_scrub
    Those two are set immediately after bbio allocation and no more
    writes after allocation, thus they are very safe.

  * async_csum
  * can_use_append
    Those two are set for each split range, and after that there is no
    writes into those two members in different threads, thus they are
    also safe.

  And there are spaces for 4 more bits before increasing the size of
  btrfs_bio again, which should be future proof enough.

- Reorder the structure members
  Now we always put the largest member first (after the huge 120 bytes
  union), making it easier to fill any holes.

This reduce the size of btrfs_bio by 8 bytes, from 312 bytes to 304 bytes.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>