[SK_BUFF]: Introduce skb_mac_header()

For the places where we need a pointer to the mac header, it is still legal to
touch skb->mac.raw directly if just adding to, subtracting from or setting it
to another layer header.

This one also converts some more cases to skb_reset_mac_header() that my
regex missed as it had no spaces before nor after '=', ugh.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 1ec05bd..f5e019c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3633,7 +3633,7 @@
 		if (!nskb)
 			return;
 
-		skb_set_mac_header(nskb, skb->mac.raw - skb->head);
+		skb_set_mac_header(nskb, skb_mac_header(skb) - skb->head);
 		nskb->nh.raw = nskb->data + (skb->nh.raw - skb->head);
 		nskb->h.raw = nskb->data + (skb->h.raw - skb->head);