[SK_BUFF]: Introduce skb_network_offset()

For the quite common 'skb->nh.raw - skb->data' sequence.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 99cd90c..669f5d9 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -96,7 +96,7 @@
 static int ip_dev_loopback_xmit(struct sk_buff *newskb)
 {
 	skb_reset_mac_header(newskb);
-	__skb_pull(newskb, newskb->nh.raw - newskb->data);
+	__skb_pull(newskb, skb_network_offset(newskb));
 	newskb->pkt_type = PACKET_LOOPBACK;
 	newskb->ip_summed = CHECKSUM_UNNECESSARY;
 	BUG_TRAP(newskb->dst);
@@ -1199,7 +1199,7 @@
 
 	/* move skb->data to ip header from ext header */
 	if (skb->data < skb->nh.raw)
-		__skb_pull(skb, skb->nh.raw - skb->data);
+		__skb_pull(skb, skb_network_offset(skb));
 	while ((tmp_skb = __skb_dequeue(&sk->sk_write_queue)) != NULL) {
 		__skb_pull(tmp_skb, skb->h.raw - skb->nh.raw);
 		*tail_skb = tmp_skb;