veth: Avoid NAPI scheduling on failed SKB forwarding

When an skb fails to be forwarded to the peer(e.g., skb data buffer
length exceeds MTU), it will not be added to the peer's receive queue.
Therefore, we should schedule the peer's NAPI poll function only when
skb forwarding is successful to avoid unnecessary overhead.
Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
Link: https://lore.kernel.org/r/20230824123131.7673-1-liangchen.linux@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/veth.c

@@ -373,14 +373,13 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (likely(veth_forward_skb(rcv, skb, rq, use_napi) == NET_RX_SUCCESS)) {
 		if (!use_napi)
 			dev_lstats_add(dev, length);
+		else
+			__veth_xdp_flush(rq);
 	} else {
 drop:
 		atomic64_inc(&priv->dropped);
 	}
 
-	if (use_napi)
-		__veth_xdp_flush(rq);
-
 	rcu_read_unlock();
 
 	return NETDEV_TX_OK;