cgroup: fix display of forceidle time at root

We need to reset forceidle_sum to 0 when reading from root, since the
bstat we accumulate into is stack allocated.

To make this more robust, just replace the existing cputime reset with a
memset of the overall bstat.

Signed-off-by: Josh Don <joshdon@google.com>
Fixes: 1fcf54deb7 ("sched/core: add forced idle accounting for cgroups")
Cc: stable@vger.kernel.org # v6.0+
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/rstat.c

@@ -457,9 +457,7 @@ static void root_cgroup_cputime(struct cgroup_base_stat *bstat)
 	struct task_cputime *cputime = &bstat->cputime;
 	int i;
 
-	cputime->stime = 0;
-	cputime->utime = 0;
-	cputime->sum_exec_runtime = 0;
+	memset(bstat, 0, sizeof(*bstat));
 	for_each_possible_cpu(i) {
 		struct kernel_cpustat kcpustat;
 		u64 *cpustat = kcpustat.cpustat;