btrfs: prepare extent_io.c for future large folio support

When we're handling folios from filemap, we can no longer assume all
folios are page sized.

Thus for call sites assuming the folio is page sized, change the
PAGE_SIZE usage to folio_size() instead.

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>