[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/include/net/udplite.h b/include/net/udplite.h
index d99df75..76503203 100644
--- a/include/net/udplite.h
+++ b/include/net/udplite.h
@@ -101,14 +101,14 @@
 
 static inline __wsum udplite_csum_outgoing(struct sock *sk, struct sk_buff *skb)
 {
-	int off, len, cscov = udplite_sender_cscov(udp_sk(sk), skb->h.uh);
+	int cscov = udplite_sender_cscov(udp_sk(sk), skb->h.uh);
 	__wsum csum = 0;
 
 	skb->ip_summed = CHECKSUM_NONE;     /* no HW support for checksumming */
 
 	skb_queue_walk(&sk->sk_write_queue, skb) {
-		off = skb->h.raw - skb->data;
-		len = skb->len - off;
+		const int off = skb_transport_offset(skb);
+		const int len = skb->len - off;
 
 		csum = skb_checksum(skb, off, (cscov > len)? len : cscov, csum);