vhost: don't bother with copying iovec in handle_tx()

just advance the msg.msg_iter and be done with that.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: kvm@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/vhost/net.c

@@ -336,7 +336,7 @@ static void handle_tx(struct vhost_net *net)
 {
 	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
 	struct vhost_virtqueue *vq = &nvq->vq;
-	unsigned out, in, s;
+	unsigned out, in;
 	int head;
 	struct msghdr msg = {
 		.msg_name = NULL,
@@ -395,16 +395,17 @@ static void handle_tx(struct vhost_net *net)
 			break;
 		}
 		/* Skip header. TODO: support TSO. */
-		s = move_iovec_hdr(vq->iov, nvq->hdr, hdr_size, out);
 		len = iov_length(vq->iov, out);
 		iov_iter_init(&msg.msg_iter, WRITE, vq->iov, out, len);
+		iov_iter_advance(&msg.msg_iter, hdr_size);
 		/* Sanity check */
-		if (!len) {
+		if (!iov_iter_count(&msg.msg_iter)) {
 			vq_err(vq, "Unexpected header len for TX: "
 			       "%zd expected %zd\n",
-			       iov_length(nvq->hdr, s), hdr_size);
+			       len, hdr_size);
 			break;
 		}
+		len = iov_iter_count(&msg.msg_iter);
 
 		zcopy_used = zcopy && len >= VHOST_GOODCOPY_LEN
 				   && (nvq->upend_idx + 1) % UIO_MAXIOV !=