btrfs: disguise single-data-RAID56 as RAID1/RAID1C3

[CHANGELOG]
RFC->v1:
- Remove unused btrfs_chunk_map::io_(align|width) members
  To make space for the new member.

- Fix a duplicated bg->flags assignment in fill_dummy_bgs()
  Just exposed by the last patch which also touches that code.

- Use a dedicated member to record the on-disk chunk type
  So it's less fragile than the RFC patch.

The single-data-RAID56 (2 disks raid5 or 3 disks raid6) is not a feature
that should be supported in the first place, thus raid56 is going to
remove such support.

Meanwhile rejecting single-data-RAID56 will cause existing btrfs users
unable to mount their fs, even if there is only a single
empty chunk with that flag, mostly caused by some degraded mount.

So to avoid impacting existing users, implement an internal
single-data-RAID56 to RAID1/RAID1C3 mapping, to co-operate with the
upstream raid56 lib changes.

Qu Wenruo (3):
  btrfs: remove btrfs_chunk_map::io_(align|width) members
  btrfs: remove duplicated block group type assignment
  btrfs: disguise single-data-RAID56 as RAID1/RAID1C3

 fs/btrfs/block-group.c |  3 +--
 fs/btrfs/volumes.c     | 25 ++++++++++++++++++-------
 fs/btrfs/volumes.h     | 10 ++++++++--
 3 files changed, 27 insertions(+), 11 deletions(-)

-- 
2.54.0