bch_btree_node_get
r->b = bch_btree_node_get(b->c, op, k, b->level - 1,
struct btree *bch_btree_node_get(struct cache_set *c, struct btree_op *op,
struct btree *_child = bch_btree_node_get((b)->c, op, key, l, \
c->root = bch_btree_node_get(c, NULL, k,