block: fix NULL pointer dereference in register_disk

If __device_add_disk-->bdi_register_owner-->bdi_register-->
bdi_register_va-->device_create_vargs fails, bdi->dev is still
NULL, __device_add_disk-->register_disk will visit bdi->dev->kobj.
This patch fixes that.
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -655,10 +655,12 @@ static void register_disk(struct device *parent, struct gendisk *disk,
 		kobject_uevent(&part_to_dev(part)->kobj, KOBJ_ADD);
 	disk_part_iter_exit(&piter);
 
+	if (disk->queue->backing_dev_info->dev) {
 		err = sysfs_create_link(&ddev->kobj,
 			  &disk->queue->backing_dev_info->dev->kobj,
 			  "bdi");
 		WARN_ON(err);
+	}
 }
 
 /**