virtio_blk: fix race at module removal

If a device appears while module is being removed,
driver will get a callback after we've given up
on the major number.

In theory this means this major number can get reused
by something else, resulting in a conflict.

To fix, cleanup in reverse order of initialization.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/block/virtio_blk.c

@@ -871,8 +871,8 @@ static int __init init(void)
 
 static void __exit fini(void)
 {
-	unregister_blkdev(major, "virtblk");
 	unregister_virtio_driver(&virtio_blk);
+	unregister_blkdev(major, "virtblk");
 	destroy_workqueue(virtblk_wq);
 }
 module_init(init);