workqueue: fix bugs in wq_update_unbound_numa() failure path

wq_update_unbound_numa() failure path has the following two bugs.

- alloc_unbound_pwq() is called without holding wq->mutex; however, if
  the allocation fails, it jumps to out_unlock which tries to unlock
  wq->mutex.

- The function should switch to dfl_pwq on failure but didn't do so
  after alloc_unbound_pwq() failure.

Fix it by regrabbing wq->mutex and jumping to use_dfl_pwq on
alloc_unbound_pwq() failure.
Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
Fixes: 4c16bd32 ("workqueue: implement NUMA affinity for unbound workqueues")

kernel/workqueue.c

@@ -4100,7 +4100,8 @@ static void wq_update_unbound_numa(struct workqueue_struct *wq, int cpu,
 	if (!pwq) {
 		pr_warning("workqueue: allocation failed while updating NUMA affinity of \"%s\"\n",
 			   wq->name);
-		goto out_unlock;
+		mutex_lock(&wq->mutex);
+		goto use_dfl_pwq;
 	}
 
 	/*