sched/fair: remove cpu.qos_leve from root cpu cgroup

jingdong inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7R399

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/sys/fs/cgroup/cpu/cpu.qos_level is not needed, and easy
to cause misunderstanding.
Signed-off-by: Nzhangwei123171 <zhangwei123171@jd.com>
Reviewed-by: Nzhaoxiaoqiang11 <zhaoxiaoqiang11@jd.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,
 	},