[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/core/dev.c b/net/core/dev.c
index 99f1572..f7f7e56 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1156,7 +1156,7 @@
 int skb_checksum_help(struct sk_buff *skb)
 {
 	__wsum csum;
-	int ret = 0, offset = skb->h.raw - skb->data;
+	int ret = 0, offset = skb_transport_offset(skb);
 
 	if (skb->ip_summed == CHECKSUM_COMPLETE)
 		goto out_set_summed;