workqueue: Use TASK_IDLE

Workqueues don't use signals, it (ab)uses TASK_INTERRUPTIBLE to avoid
increasing the loadavg numbers. We've 'recently' introduced TASK_IDLE
for this case:

  80ed87c8 ("sched/wait: Introduce TASK_NOLOAD and TASK_IDLE")

use it.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -2247,7 +2247,7 @@ static int worker_thread(void *__worker)
 	 * event.
 	 */
 	worker_enter_idle(worker);
-	__set_current_state(TASK_INTERRUPTIBLE);
+	__set_current_state(TASK_IDLE);
 	spin_unlock_irq(&pool->lock);
 	schedule();
 	goto woke_up;
@@ -2289,7 +2289,7 @@ static int rescuer_thread(void *__rescuer)
 	 */
 	rescuer->task->flags |= PF_WQ_WORKER;
 repeat:
-	set_current_state(TASK_INTERRUPTIBLE);
+	set_current_state(TASK_IDLE);
 
 	/*
 	 * By the time the rescuer is requested to stop, the workqueue