!1660 remove cpu.qos_leve from root cpu cgroup

Merge Pull Request from: @zhangwei123171 
 


/sys/fs/cgroup/cpu/cpu.qos_level is not needed, and easy to cause misunderstanding. 
 
Link:https://gitee.com/openeuler/kernel/pulls/1660 

Reviewed-by: zhao xiaoqiang <zhaoxiaoqiang11@jd.com> 
Reviewed-by: Zucheng Zheng <zhengzucheng@huawei.com> 
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com> 
Acked-by: Xie XiuQi <xiexiuqi@huawei.com> 

kernel/sched/core.c

@@ -9666,6 +9666,7 @@ static struct cftype cpu_legacy_files[] = {
 #ifdef CONFIG_QOS_SCHED
 	{
 		.name = "qos_level",
+		.flags = CFTYPE_NOT_ON_ROOT,
 		.read_s64 = cpu_qos_read,
 		.write_s64 = cpu_qos_write,
 	},