sched: Fix update_curr_rt()

cpu_stopper_thread()
  migration_cpu_stop()
    __migrate_task()
      deactivate_task()
        dequeue_task()
          dequeue_task_rq()
            update_curr_rt()

Will call update_curr_rt() on rq->curr, which at that time is
rq->stop. The problem is that rq->stop.prio matches an RT prio and
thus falsely assumes its a rt_sched_class task.
Reported-Debuged-Tested-Acked-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Cc: stable@kernel.org # .37
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched_rt.c

@@ -625,7 +625,7 @@ static void update_curr_rt(struct rq *rq)
 	struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
 	u64 delta_exec;
 
-	if (!task_has_rt_policy(curr))
+	if (curr->sched_class != &rt_sched_class)
 		return;
 
 	delta_exec = rq->clock_task - curr->se.exec_start;