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>
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c273376..154aa12 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3750,7 +3750,7 @@
 
 	if (pwq) {
 		put_unbound_pool(pwq->pool);
-		kfree(pwq);
+		kmem_cache_free(pwq_cache, pwq);
 	}
 }