l2tp: fix a potential skb leak in l2tp_xmit_skb()

l2tp_xmit_skb() can leak one skb if skb_cow_head() returns an error.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -1045,8 +1045,10 @@ int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb, int hdr_len
 	headroom = NET_SKB_PAD + sizeof(struct iphdr) +
 		uhlen + hdr_len;
 	old_headroom = skb_headroom(skb);
-	if (skb_cow_head(skb, headroom))
+	if (skb_cow_head(skb, headroom)) {
+		dev_kfree_skb(skb);
 		goto abort;
+	}
 
 	new_headroom = skb_headroom(skb);
 	skb_orphan(skb);