[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/ipv6/icmp.c b/net/ipv6/icmp.c
index a91dfbc..aa4a0a5 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -206,7 +206,7 @@
 {
 	u8 _optval, *op;
 
-	offset += skb->nh.raw - skb->data;
+	offset += skb_network_offset(skb);
 	op = skb_header_pointer(skb, offset, sizeof(_optval), &_optval);
 	if (op == NULL)
 		return 1;
@@ -431,7 +431,7 @@
 		tclass = 0;
 
 	msg.skb = skb;
-	msg.offset = skb->nh.raw - skb->data;
+	msg.offset = skb_network_offset(skb);
 	msg.type = type;
 
 	len = skb->len - msg.offset;