cpuset: fix RCU lockdep splat in cpuset_print_task_mems_allowed()

Sasha reported a lockdep warning when OOM was triggered. The reason
is cgroup_name() should be called with rcu_read_lock() held.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cpuset.c

@@ -2599,6 +2599,7 @@ void cpuset_print_task_mems_allowed(struct task_struct *tsk)
 
 	struct cgroup *cgrp = task_cs(tsk)->css.cgroup;
 
+	rcu_read_lock();
 	spin_lock(&cpuset_buffer_lock);
 
 	nodelist_scnprintf(cpuset_nodelist, CPUSET_NODELIST_LEN,
@@ -2607,6 +2608,7 @@ void cpuset_print_task_mems_allowed(struct task_struct *tsk)
 	       tsk->comm, cgroup_name(cgrp), cpuset_nodelist);
 
 	spin_unlock(&cpuset_buffer_lock);
+	rcu_read_unlock();
 }
 
 /*