vdpa: Mark vdpa_config_ops.get_vq_notification as optional

mainline inclusion
from mainline-v5.17-rc1
commit 28cc408b
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=28cc408be72cebb0f3fcc37bc74ab3196d4de726

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Since vhost_vdpa_mmap checks for its existence before calling it.
Signed-off-by: NEugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20211104195248.2088904-1-eperezma@redhat.comSigned-off-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Reviewed-by: NStefano Garzarella <sgarzare@redhat.com>
Signed-off-by: NPengyuan Zhao <zhaopengyuan@hisilicon.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