dm delay: fix a crash when invalid device is specified

commit 81bc6d15 upstream.

When the target line contains an invalid device, delay_ctr() will call
delay_dtr() with NULL workqueue.  Attempting to destroy the NULL
workqueue causes a crash.
Signed-off-by: NMikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

drivers/md/dm-delay.c

@@ -121,7 +121,8 @@ static void delay_dtr(struct dm_target *ti)
 {
 	struct delay_c *dc = ti->private;
 
-	destroy_workqueue(dc->kdelayd_wq);
+	if (dc->kdelayd_wq)
+		destroy_workqueue(dc->kdelayd_wq);
 
 	if (dc->read.dev)
 		dm_put_device(ti, dc->read.dev);