btrfs: prepare btrfs_page_mkwrite() for large folios

This changes the assumption that the folio is always page sized.
(Although the ASSERT() for folio order is still kept as-is).

Just replace the PAGE_SIZE with folio_size().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>