perf_event: Clean up perf_event_init_task()

While at it: we can traverse ctx->group_list to get all
group leader, it should be safe since we hold ctx->mutex.

Changlog v1->v2:

  - remove WARN_ON_ONCE() according to Peter Zijlstra's suggestion
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <4ABC5AF9.6060808@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -4767,9 +4767,7 @@ int perf_event_init_task(struct task_struct *child)
 	 * We dont have to disable NMIs - we are only looking at
 	 * the list, not manipulating it:
 	 */
-	list_for_each_entry_rcu(event, &parent_ctx->event_list, event_entry) {
-		if (event != event->group_leader)
-			continue;
+	list_for_each_entry(event, &parent_ctx->group_list, group_entry) {
 
 		if (!event->attr.inherit) {
 			inherited_all = 0;