cgroup: properly init u64_stats

Lockdep complains that the stats update is trying to register a non-static
key. This is because u64_stats are using a seqlock on 32bit arches, which
needs to be initialized before usage.

Fixes: 041cd640 (cgroup: Implement cgroup2 basic CPU usage accounting)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/stat.c

@@ -296,8 +296,12 @@ int cgroup_stat_init(struct cgroup *cgrp)
 	}
 
 	/* ->updated_children list is self terminated */
-	for_each_possible_cpu(cpu)
-		cgroup_cpu_stat(cgrp, cpu)->updated_children = cgrp;
+	for_each_possible_cpu(cpu) {
+		struct cgroup_cpu_stat *cstat = cgroup_cpu_stat(cgrp, cpu);
+
+		cstat->updated_children = cgrp;
+		u64_stats_init(&cstat->sync);
+	}
 
 	prev_cputime_init(&cgrp->stat.prev_cputime);