sched: fix the task_group hierarchy for UID grouping

UID grouping doesn't actually have a task_group representing the root of
the task_group tree. Add one.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/user.c b/kernel/user.c
index a28d9f9..debce60 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -101,7 +101,7 @@
 {
 	int rc = 0;
 
-	up->tg = sched_create_group(NULL);
+	up->tg = sched_create_group(&root_task_group);
 	if (IS_ERR(up->tg))
 		rc = -ENOMEM;