cgroup: Fix task_css_check rcu warnings

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4OF4N

------------------------------------------

task_css() should be protected by rcu. In my environment
if not protected by rcu qemu may fail to start.

Fixes: a885e3f9 ("psi: support psi under cgroup v1")
Reported-by: NYang Yingliang <yangyingliang@huawei.com>
Acked-by: NMichael Wang <yun.wany@linux.alibaba.com>
Signed-off-by: NXunlei Pang <xlpang@linux.alibaba.com>
Signed-off-by: NYihao Wu <wuyihao@linux.alibaba.com>
Acked-by: NYang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: NChen Wandun <chenwandun@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

kernel/sched/psi.c

@@ -755,12 +755,15 @@ static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
 	if (!*iter) {
 		if (static_branch_likely(&psi_v1_disabled))
 			cgroup = task->cgroups->dfl_cgrp;
-		else
+		else {
 #ifdef CONFIG_CGROUP_CPUACCT
+			rcu_read_lock();
 			cgroup = task_cgroup(task, cpuacct_cgrp_id);
+			rcu_read_unlock();
 #else
 			cgroup = NULL;
 #endif
+		}
 	} else if (*iter == &psi_system)
 		return NULL;
 	else