workqueue: invert the order between pool->lock and wq_mayday_lock

Currently, pool->lock nests inside pool->lock.  There's no inherent
reason for this order.  The only place where the two locks are held
together is pool_mayday_timeout() and it just got decided that way.

This nesting order turns out to complicate things with the planned
rescuer_thread() update.  Let's invert them.  This doesn't cause any
behavior differences.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: NeilBrown <neilb@suse.de>
Cc: Dongsu Park <dongsu.park@profitbricks.com>

kernel/workqueue.c

@@ -1804,8 +1804,8 @@ static void pool_mayday_timeout(unsigned long __pool)
 	struct worker_pool *pool = (void *)__pool;
 	struct work_struct *work;
 
-	spin_lock_irq(&wq_mayday_lock);		/* for wq->maydays */
-	spin_lock(&pool->lock);
+	spin_lock_irq(&pool->lock);
+	spin_lock(&wq_mayday_lock);		/* for wq->maydays */
 
 	if (need_to_create_worker(pool)) {
 		/*
@@ -1818,8 +1818,8 @@ static void pool_mayday_timeout(unsigned long __pool)
 			send_mayday(work);
 	}
 
-	spin_unlock(&pool->lock);
-	spin_unlock_irq(&wq_mayday_lock);
+	spin_unlock(&wq_mayday_lock);
+	spin_unlock_irq(&pool->lock);
 
 	mod_timer(&pool->mayday_timer, jiffies + MAYDAY_INTERVAL);
 }