vhost-net: initialize zcopy packet counters

These packet counters are used to drive the zercopy
selection heuristic so nothing too bad happens if they are off a bit -
and they are also reset once in a while.
But it's cleaner to clear them when backend is set so that
we start in a known state.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/net.c

@@ -823,6 +823,9 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 		r = vhost_init_used(vq);
 		if (r)
 			goto err_vq;
+
+		n->tx_packets = 0;
+		n->tx_zcopy_err = 0;
 	}
 
 	mutex_unlock(&vq->mutex);