[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/drivers/net/skge.c b/drivers/net/skge.c
index d476a3c..ca7a0e0 100644
--- a/drivers/net/skge.c
+++ b/drivers/net/skge.c
@@ -2654,7 +2654,7 @@
 	td->dma_hi = map >> 32;
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL) {
-		int offset = skb->h.raw - skb->data;
+		const int offset = skb_transport_offset(skb);
 
 		/* This seems backwards, but it is what the sk98lin
 		 * does.  Looks like hardware is wrong?