[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/core/dev.c b/net/core/dev.c
index 424d6d0..2fcaf5b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1066,7 +1066,7 @@
 			   set by sender, so that the second statement is
 			   just protection against buggy protocols.
 			 */
-			skb2->mac.raw = skb2->data;
+			skb_reset_mac_header(skb2);
 
 			if (skb2->nh.raw < skb2->data ||
 			    skb2->nh.raw > skb2->tail) {
@@ -1206,7 +1206,7 @@
 
 	BUG_ON(skb_shinfo(skb)->frag_list);
 
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	skb->mac_len = skb->nh.raw - skb->data;
 	__skb_pull(skb, skb->mac_len);