[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/ip_gre.c b/net/ipv4/ip_gre.c
index 9151da6..88f8aae 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -616,7 +616,7 @@
 				offset += 4;
 		}
 
-		skb->mac.raw = skb->nh.raw;
+		skb_reset_mac_header(skb);
 		skb->nh.raw = __pskb_pull(skb, offset);
 		skb_postpull_rcsum(skb, skb->h.raw, offset);
 		skb->pkt_type = PACKET_HOST;