workqueue: use kmem_cache_free() instead of kfree()

memory allocated by kmem_cache_alloc() should be freed using
kmem_cache_free(), not kfree().
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3750,7 +3750,7 @@ static void free_unbound_pwq(struct pool_workqueue *pwq)
 
 	if (pwq) {
 		put_unbound_pool(pwq->pool);
-		kfree(pwq);
+		kmem_cache_free(pwq_cache, pwq);
 	}
 }