btrfs_get_free_objectid
int btrfs_get_free_objectid(struct btrfs_root *root, u64 *objectid);
ret = btrfs_get_free_objectid(trans->fs_info->tree_root, &ino);
ret = btrfs_get_free_objectid(root, &objectid);
ret = btrfs_get_free_objectid(fs_info->tree_root, &objectid);
ret = btrfs_get_free_objectid(root, &objectid);
pending->error = btrfs_get_free_objectid(tree_root, &objectid);