bch_btree_node_alloc
struct btree *n = bch_btree_node_alloc(b->c, op, b->level, b->parent);
n2 = bch_btree_node_alloc(b->c, op, b->level, b->parent);
n3 = bch_btree_node_alloc(b->c, op, b->level + 1, NULL);