net: revert "net: l2tp: Treat NET_XMIT_CN as success in l2tp_eth_dev_xmit"

This reverts commit 7c6ae610a1f0, because l2tp_xmit_skb() never
returns NET_XMIT_CN, it ignores the return value of l2tp_xmit_core().

Cc: Gao Feng <gfree.wind@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_eth.c

@@ -97,7 +97,7 @@ static int l2tp_eth_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 	unsigned int len = skb->len;
 	int ret = l2tp_xmit_skb(session, skb, session->hdr_len);
 
-	if (likely(ret == NET_XMIT_SUCCESS || ret == NET_XMIT_CN)) {
+	if (likely(ret == NET_XMIT_SUCCESS)) {
 		atomic_long_add(len, &priv->tx_bytes);
 		atomic_long_inc(&priv->tx_packets);
 	} else {