btrfs: subpage: make btrfs_is_subpage() check against a folio

To support large data folios, we can no longer assume every filemap
folio is page sized.

So btrfs_is_subpage() check must be done against a folio.

Thankfully for metadata folios, we have the full control and ensure a
large folio will not be large than nodesize, so
btrfs_meta_is_subpage() doesn't need this change.

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>