btrfs: rename btrfs_release_extent_buffer_pages() to mention folios

Continue page to folio updates, sync what the function does with it's
name.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>