btrfs_backref_iter_next
ret = btrfs_backref_iter_next(trans->fs_info, iter);
for (; ret == 0; ret = btrfs_backref_iter_next(trans->fs_info, iter)) {
int btrfs_backref_iter_next(struct btrfs_fs_info *fs_info, struct btrfs_backref_iter *iter);