btrfs: replace PAGE_SIZE with folio_size for subpage.[ch]

Since we can no longer assume all data filemap folios are page sized,
use proper folio_size() calls to determine the folio size, as a
preparation for future large data filemap folios.

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>