sort_cmp
heap_sift(iter, i, b->ops->sort_cmp);
k = __bch_btree_iter_next(iter, b->ops->sort_cmp);
bool (*sort_cmp)(struct btree_iter_set l,
.sort_cmp = bch_key_sort_cmp,
.sort_cmp = bch_extent_sort_cmp,
sort(vec, n, sizeof(struct aa_profile *), sort_cmp, NULL);