[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/sk98lin/skge.c b/drivers/net/sk98lin/skge.c
index 9ac1fe6..e4ab7a8 100644
--- a/drivers/net/sk98lin/skge.c
+++ b/drivers/net/sk98lin/skge.c
@@ -1562,7 +1562,7 @@
 	pTxd->pMBuf     = pMessage;
 
 	if (pMessage->ip_summed == CHECKSUM_PARTIAL) {
-		u16 hdrlen = pMessage->h.raw - pMessage->data;
+		u16 hdrlen = skb_transport_offset(pMessage);
 		u16 offset = hdrlen + pMessage->csum_offset;
 
 		if ((pMessage->h.ipiph->protocol == IPPROTO_UDP ) &&
@@ -1681,7 +1681,7 @@
 	** Does the HW need to evaluate checksum for TCP or UDP packets? 
 	*/
 	if (pMessage->ip_summed == CHECKSUM_PARTIAL) {
-		u16 hdrlen = pMessage->h.raw - pMessage->data;
+		u16 hdrlen = skb_transport_offset(pMessage);
 		u16 offset = hdrlen + pMessage->csum_offset;
 
 		Control = BMU_STFWD;