Fix crash in virtio_blk during modprobe ; rmmod ; modprobe

Fix a modprobe virtio_blk ; rmmod virtio_blk ; modprobe virtio_blk crash; this
was basically because we weren't doing "del_gendisk()" in the remove path.
Signed-off-by: NChris Lalancette <clalance@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (moved del_gendisk up)

drivers/block/virtio_blk.c

@@ -311,6 +311,7 @@ static void virtblk_remove(struct virtio_device *vdev)
 	/* Stop all the virtqueues. */
 	vdev->config->reset(vdev);
 
+	del_gendisk(vblk->disk);
 	blk_cleanup_queue(vblk->disk->queue);
 	put_disk(vblk->disk);
 	mempool_destroy(vblk->pool);