[SK_BUFF]: Introduce skb_reset_mac_header(skb)

For the common, open coded 'skb->mac.raw = skb->data' operation, so that we can
later turn skb->mac.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>
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 0b3d7bf..29ee7be 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2747,7 +2747,8 @@
 	/* Reserve room for dummy headers, this skb can pass
 	   through good chunk of routing engine.
 	 */
-	skb->mac.raw = skb->nh.raw = skb->data;
+	skb_reset_mac_header(skb);
+	skb->nh.raw = skb->data;
 
 	/* Bugfix: need to give ip_route_input enough of an IP header to not gag. */
 	skb->nh.iph->protocol = IPPROTO_ICMP;