tun: return NET_XMIT_DROP for dropped packets

After commit 5d097109257c03a71845729f8db6b5770c4bbedc
("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 <[e-mail]>
Signed-off-by: Jason Wang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tun.c

···
 	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)