vdpa: fix not using CVQ buffer in case of error

Bug introducing when refactoring.  Otherway, the guest never received
the used buffer.

Fixes: be4278b ("vdpa: extract vhost_vdpa_net_cvq_add from vhost_vdpa_net_handle_ctrl_avail")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20230602173451.1917999-1-eperezma@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>

net/vhost-vdpa.c

@@ -794,7 +794,7 @@ static int vhost_vdpa_net_handle_ctrl_avail(VhostShadowVirtqueue *svq,
     }
 
     if (*s->status != VIRTIO_NET_OK) {
-        return VIRTIO_NET_ERR;
+        goto out;
     }
 
     status = VIRTIO_NET_ERR;