cgroup: use release_agent_path_lock in cgroup_release_agent_show()

release_path is now protected by release_agent_path_lock to allow
accessing it without grabbing cgroup_mutex; however,
cgroup_release_agent_show() was still grabbing cgroup_mutex.  Let's
convert it to release_agent_path_lock so that we don't have to worry
about this one for the planned locking updates.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -2373,11 +2373,10 @@ static int cgroup_release_agent_show(struct seq_file *seq, void *v)
 {
 	struct cgroup *cgrp = seq_css(seq)->cgroup;
 
-	if (!cgroup_lock_live_group(cgrp))
-		return -ENODEV;
+	spin_lock(&release_agent_path_lock);
 	seq_puts(seq, cgrp->root->release_agent_path);
+	spin_unlock(&release_agent_path_lock);
 	seq_putc(seq, '\n');
-	mutex_unlock(&cgroup_mutex);
 	return 0;
 }