vhost: allow batching hint without size

Commit e2ae38cf ("vhost: fix hung thread due to erroneous iotlb
entries") tries to reject the IOTLB message whose size is zero. But
the size is not necessarily meaningful, one example is the batching
hint, so the commit breaks that.

Fixing this be reject zero size message only if the message is used to
update/invalidate the IOTLB.

Fixes: e2ae38cf ("vhost: fix hung thread due to erroneous iotlb entries")
Reported-by: NEli Cohen <elic@nvidia.com>
Cc: Anirudh Rayabharam <mail@anirudhrb.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20220310075211.4801-1-jasowang@redhat.comSigned-off-by: NMichael S. Tsirkin <mst@redhat.com>
Tested-by: NEli Cohen <elic@nvidia.com>

drivers/vhost/vhost.c

@@ -1170,7 +1170,9 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
 		goto done;
 	}
 
-	if (msg.size == 0) {
+	if ((msg.type == VHOST_IOTLB_UPDATE ||
+	     msg.type == VHOST_IOTLB_INVALIDATE) &&
+	     msg.size == 0) {
 		ret = -EINVAL;
 		goto done;
 	}