workqueue: use system_highpri_wq for unbind_work

To speed cpu down processing up, use system_highpri_wq.
As scheduling priority of workers on it is higher than system_wq and
it is not contended by other normal works on this cpu, work on it
is processed faster than system_wq.

tj: CPU up/downs care quite a bit about latency these days.  This
    shouldn't hurt anything and makes sense.
Signed-off-by: NJoonsoo Kim <js1304@gmail.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3680,7 +3680,7 @@ static int __devinit workqueue_cpu_down_callback(struct notifier_block *nfb,
 	case CPU_DOWN_PREPARE:
 		/* unbinding should happen on the local CPU */
 		INIT_WORK_ONSTACK(&unbind_work, gcwq_unbind_fn);
-		schedule_work_on(cpu, &unbind_work);
+		queue_work_on(cpu, system_highpri_wq, &unbind_work);
 		flush_work(&unbind_work);
 		break;
 	}