[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/sctp/input.c b/net/sctp/input.c
index 595fe32..9311b5d 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c
@@ -141,7 +141,7 @@
 	sh = (struct sctphdr *) skb->h.raw;
 
 	/* Pull up the IP and SCTP headers. */
-	__skb_pull(skb, skb->h.raw - skb->data);
+	__skb_pull(skb, skb_transport_offset(skb));
 	if (skb->len < sizeof(struct sctphdr))
 		goto discard_it;
 	if ((skb->ip_summed != CHECKSUM_UNNECESSARY) &&