btrfs: separate/simplify/unify subpage handling

[CHANGELOG]
v2:
- Fix a crash in 4K page sized system caused by
  btrfs_clear_buffer_dirty()
  It looks like using btrfs_meta_folio_set_writeback() and
  btrfs_meta_folio_clear_writeback() is screwing up the btree inode map.
  
  Fix it by reverting to the old manually clearing of
  PAGECACHE_DIRTY_TAG.

[METADATA SUBPAGE CHECKS]
Currently we only have one btrfs_is_subpage() check, utilized by data
and metadata.

But the truth is, btrfs_is_subpage() can return incorrect result if the
target folio belongs to a dummy extent buffer.

This means we have quite some metadata call sites doing their own
metadata specific subpage check.

[SUBPAGE SPECIFIC HANDLINGS]
There are several functions that split the metadata subpage handling
into a dedicated path.

But the truth is, a lot of such paths only have minor differences
compared to the regular routine.

[THIS SERIES]
This series address the above problems and prepare for the incoming data
larger folio support by:

- Remove btrfs_fs_info::sectors_per_page
  This is mostly a preparation for the future data larger folio support.

- Introduce a dedicated metadata specific subpage check

- Unify/simplify metadata subpage handling
  So that we have a single unified path for metadata

Qu Wenruo (8):
  btrfs: remove btrfs_fs_info::sectors_per_page
  btrfs: extract metadata subpage detection into a dedicated helper
  btrfs: make subpage attach and detach to handle metadata properly
  btrfs: use metadata specific helpers to simplify extent buffer helpers
  btrfs: simplify btrfs_clear_buffer_dirty()
  btrfs: simplify write_one_eb()
  btrfs: simplify read_extent_buffer_pages_nowait()
  btrfs: require strict data/metadata split for subpage check

 fs/btrfs/disk-io.c   |   5 +-
 fs/btrfs/extent_io.c | 184 +++++++++++++++-------------------------
 fs/btrfs/fs.h        |   7 +-
 fs/btrfs/subpage.c   | 196 +++++++++++++++++++++++++++----------------
 fs/btrfs/subpage.h   |  52 ++++++++++--
 5 files changed, 247 insertions(+), 197 deletions(-)

-- 
2.48.1