bch_btree_iter_stack_init
bch_btree_iter_stack_init(b, &iter, NULL);
m = bch_btree_iter_stack_init(b, &iter, preceding_key_p);
struct bkey *bch_btree_iter_stack_init(struct btree_keys *b,
for (bch_btree_iter_stack_init((b), (stack_iter), NULL); \
for (bch_btree_iter_stack_init((b), (stack_iter), NULL); \
bch_btree_iter_stack_init(&b->keys, &iter, &b->c->gc_done);
bch_btree_iter_stack_init(&b->keys, &iter, NULL);
bch_btree_iter_stack_init(&c->root->keys, &iter, NULL);
bch_btree_iter_stack_init(&b->keys, &iter, from);
bch_btree_iter_stack_init(&b->keys, &iter, from);
bch_btree_iter_stack_init(&c->root->keys, &iter, NULL);