cgroup: fix missing unlock on error in cgroup_load_subsys()

Add the missing unlock before return from function cgroup_load_subsys()
in the error handling case.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -4618,6 +4618,7 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
 	if (IS_ERR(css)) {
 		/* failure case - need to deassign the cgroup_subsys[] slot. */
 		cgroup_subsys[ss->subsys_id] = NULL;
+		mutex_unlock(&cgroup_root_mutex);
 		mutex_unlock(&cgroup_mutex);
 		return PTR_ERR(css);
 	}