veth: Account for packet drops in ndo_xdp_xmit

Use existing atomic drop counter. Since drop path is really an
exceptional case here, I'm thinking atomic ops would not hurt the
performance.
XDP packets and bytes are not counted in ndo_xdp_xmit, but will be
accounted on rx side by the following commit.
Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/veth.c

@@ -308,16 +308,20 @@ static int veth_xdp_xmit(struct net_device *dev, int n,
 {
 	struct veth_priv *rcv_priv, *priv = netdev_priv(dev);
 	struct net_device *rcv;
+	int i, ret, drops = n;
 	unsigned int max_len;
 	struct veth_rq *rq;
-	int i, drops = 0;
 
-	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK))
-		return -EINVAL;
+	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK)) {
+		ret = -EINVAL;
+		goto drop;
+	}
 
 	rcv = rcu_dereference(priv->peer);
-	if (unlikely(!rcv))
-		return -ENXIO;
+	if (unlikely(!rcv)) {
+		ret = -ENXIO;
+		goto drop;
+	}
 
 	rcv_priv = netdev_priv(rcv);
 	rq = &rcv_priv->rq[veth_select_rxq(rcv)];
@@ -325,9 +329,12 @@ static int veth_xdp_xmit(struct net_device *dev, int n,
 	 * side. This means an XDP program is loaded on the peer and the peer
 	 * device is up.
 	 */
-	if (!rcu_access_pointer(rq->xdp_prog))
-		return -ENXIO;
+	if (!rcu_access_pointer(rq->xdp_prog)) {
+		ret = -ENXIO;
+		goto drop;
+	}
 
+	drops = 0;
 	max_len = rcv->mtu + rcv->hard_header_len + VLAN_HLEN;
 
 	spin_lock(&rq->xdp_ring.producer_lock);
@@ -346,7 +353,14 @@ static int veth_xdp_xmit(struct net_device *dev, int n,
 	if (flags & XDP_XMIT_FLUSH)
 		__veth_xdp_flush(rq);
 
-	return n - drops;
+	if (likely(!drops))
+		return n;
+
+	ret = n - drops;
+drop:
+	atomic64_add(drops, &priv->dropped);
+
+	return ret;
 }
 
 static void veth_xdp_flush(struct net_device *dev)