vdpa: Mark vdpa_config_ops.get_vq_notification as optional

Since vhost_vdpa_mmap checks for its existence before calling it.
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20211104195248.2088904-1-eperezma@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

include/linux/vdpa.h

@@ -157,7 +157,7 @@ struct vdpa_map_file {
  *				@vdev: vdpa device
  *				@idx: virtqueue index
  *				@state: pointer to returned state (last_avail_idx)
- * @get_vq_notification:	Get the notification area for a virtqueue
+ * @get_vq_notification:	Get the notification area for a virtqueue (optional)
  *				@vdev: vdpa device
  *				@idx: virtqueue index
  *				Returns the notifcation area