btrfs_del_root
ret = btrfs_del_root(trans, &root->root_key);
ret = btrfs_del_root(trans, &free_space_root->root_key);
ret = btrfs_del_root(trans, "a_root->root_key);
int btrfs_del_root(struct btrfs_trans_handle *trans, const struct btrfs_key *key);