tun: return NET_XMIT_DROP for dropped packets

After commit 5d097109
("tun: only queue packets on device"), NETDEV_TX_OK was returned for
dropped packets. This will confuse pktgen since dropped packets were
counted as sent ones.

Fixing this by returning NET_XMIT_DROP to let pktgen count it as error
packet.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -819,7 +819,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 	skb_tx_error(skb);
 	kfree_skb(skb);
 	rcu_read_unlock();
-	return NETDEV_TX_OK;
+	return NET_XMIT_DROP;
 }
 
 static void tun_net_mclist(struct net_device *dev)