btrfs: require strict data/metadata split for subpage checks

btrfs: require strict data/metadata split for subpage checks

Since we have btrfs_meta_is_subpage(), we should make btrfs_is_subpage()
to be data inode specific.

This change involves:

- Simplify btrfs_is_subpage()
  Now we only need to do a very simple sectorsize check against
  PAGE_SIZE.
  And since the function is pretty simple now, just make it an inline
  function.

- Add an extra ASSERT() to make sure btrfs_is_subpage() is only called
  on data inode mapping

- Migrate btree_csum_one_bio() to use btrfs_meta_folio_*() helpers
- Migrate alloc_extent_buffer() to use btrfs_meta_folio_*() helpers
- Migrate end_bbio_meta_write() to use btrfs_meta_folio_*() helpers
  Or we will trigger the ASSERT() due to calling btrfs_folio_*() on
  metadata folios.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>