[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/netrom/nr_dev.c b/net/netrom/nr_dev.c
index 9a97ed6..17c3f1e 100644
--- a/net/netrom/nr_dev.c
+++ b/net/netrom/nr_dev.c
@@ -56,7 +56,7 @@
 
 	/* Spoof incoming device */
 	skb->dev      = dev;
-	skb->mac.raw  = skb->nh.raw;
+	skb_reset_mac_header(skb);
 	skb->nh.raw   = skb->data;
 	skb->pkt_type = PACKET_HOST;