[SK_BUFF]: Introduce skb_network_offset()

For the quite common 'skb->nh.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/gianfar.c b/drivers/net/gianfar.c
index 1d01919..c7a7093 100644
--- a/drivers/net/gianfar.c
+++ b/drivers/net/gianfar.c
@@ -952,7 +952,7 @@
 	 * frame (skb->data) and the start of the IP hdr.
 	 * l4os is the distance between the start of the
 	 * l3 hdr and the l4 hdr */
-	fcb->l3os = (u16)(skb->nh.raw - skb->data - GMAC_FCB_LEN);
+	fcb->l3os = (u16)(skb_network_offset(skb) - GMAC_FCB_LEN);
 	fcb->l4os = (u16)(skb->h.raw - skb->nh.raw);
 
 	fcb->flags = flags;