static void bch2_btree_set_root(struct btree_update *as,
				struct btree_trans *trans,
				struct btree_path *path,
				struct btree *b)
static int bch2_btree_set_root(struct btree_update *as,
			       struct btree_trans *trans,
			       struct btree_path *path,
			       struct btree *b,
			       bool nofail)