vhost: fix zcopy reference counting

Fix get/put refcount imbalance with zero copy,
which caused qemu to hang forever on guest driver unload.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -1578,7 +1578,6 @@ struct vhost_ubuf_ref *vhost_ubuf_alloc(struct vhost_virtqueue *vq,
 	if (!ubufs)
 		return ERR_PTR(-ENOMEM);
 	kref_init(&ubufs->kref);
-	kref_get(&ubufs->kref);
 	init_waitqueue_head(&ubufs->wait);
 	ubufs->vq = vq;
 	return ubufs;