[SK_BUFF]: Introduce skb_transport_offset()

For the quite common 'skb->h.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/mip6.c b/net/ipv6/mip6.c
index 7b5f9d8..8520289 100644
--- a/net/ipv6/mip6.c
+++ b/net/ipv6/mip6.c
@@ -90,8 +90,9 @@
 {
 	struct ip6_mh *mh;
 
-	if (!pskb_may_pull(skb, (skb->h.raw - skb->data) + 8) ||
-	    !pskb_may_pull(skb, (skb->h.raw - skb->data) + ((skb->h.raw[1] + 1) << 3)))
+	if (!pskb_may_pull(skb, (skb_transport_offset(skb)) + 8) ||
+	    !pskb_may_pull(skb, (skb_transport_offset(skb) +
+				 ((skb->h.raw[1] + 1) << 3))))
 		return -1;
 
 	mh = (struct ip6_mh *)skb->h.raw;