[SCSI] bsg: fix bsg_unregister_queue

scsi_sysfs_add_sdev ignores the bsg_register_queue failure, so
bsg_unregister_queue must check whether the queue has a bsg device.

Signed-off-by: FUJITA Tomonori <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>

block/bsg.c

···
 {
 	struct bsg_class_device *bcd = &q->bsg_dev;
 
-	WARN_ON(!bcd->class_dev);
+	if (!bcd->class_dev)
+		return;
 
 	mutex_lock(&bsg_mutex);
 	sysfs_remove_link(&q->kobj, "bsg");