workqueue: correct handling of the pool spin_lock

When we fail to mutex_trylock(), we release the pool spin_lock and do
mutex_lock(). After that, we should regrab the pool spin_lock, but,
regrabbing is missed in current code. So correct it.

Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -2059,6 +2059,7 @@ static bool manage_workers(struct worker *worker)
 	if (unlikely(!mutex_trylock(&pool->manager_mutex))) {
 		spin_unlock_irq(&pool->lock);
 		mutex_lock(&pool->manager_mutex);
+		spin_lock_irq(&pool->lock);
 		ret = true;
 	}