[SK_BUFF]: Introduce skb_reset_network_header(skb)

For the common, open coded 'skb->nh.raw = skb->data' operation, so that we can
later turn skb->nh.raw into a offset, reducing the size of struct sk_buff in
64bit land while possibly keeping it as a pointer on 32bit.

This one touches just the most simple case, next will handle the slightly more
"complex" cases.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ax25/af_ax25.c

@@ -1548,7 +1548,7 @@ static int ax25_sendmsg(struct kiocb *iocb, struct socket *sock,
 		goto out;
 	}
 
-	skb->nh.raw = skb->data;
+	skb_reset_network_header(skb);
 
 	/* Add the PID if one is not supplied by the user in the skb */
 	if (!ax25->pidincl) {