l2tp: give proper headroom in pppol2tp_xmit()

pppol2tp_xmit() calls skb_cow_head(skb, 2) before calling
l2tp_xmit_skb()

Then l2tp_xmit_skb() calls again skb_cow_head(skb, large_headroom)

This patchs changes the first skb_cow_head() call to supply the needed
headroom to make sure at most one (expensive) pskb_expand_head() is
done.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ppp.c

@@ -395,6 +395,7 @@ static int pppol2tp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
 	struct pppol2tp_session *ps;
 	int old_headroom;
 	int new_headroom;
+	int uhlen, headroom;
 
 	if (sock_flag(sk, SOCK_DEAD) || !(sk->sk_state & PPPOX_CONNECTED))
 		goto abort;
@@ -413,7 +414,13 @@ static int pppol2tp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
 		goto abort_put_sess;
 
 	old_headroom = skb_headroom(skb);
-	if (skb_cow_head(skb, sizeof(ppph)))
+	uhlen = (tunnel->encap == L2TP_ENCAPTYPE_UDP) ? sizeof(struct udphdr) : 0;
+	headroom = NET_SKB_PAD +
+		   sizeof(struct iphdr) + /* IP header */
+		   uhlen +		/* UDP header (if L2TP_ENCAPTYPE_UDP) */
+		   session->hdr_len +	/* L2TP header */
+		   sizeof(ppph);	/* PPP header */
+	if (skb_cow_head(skb, headroom))
 		goto abort_put_sess_tun;
 
 	new_headroom = skb_headroom(skb);