[SK_BUFF]: Introduce skb_reset_transport_header(skb)

For the common, open coded 'skb->h.raw = skb->data' operation, so that we can
later turn skb->h.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 cases:

skb->h.raw = skb->data;
skb->h.raw = {skb_push|[__]skb_pull}()

The next ones 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/ah4.c b/net/ipv4/ah4.c
index 00fd31d..ebcc797 100644
--- a/net/ipv4/ah4.c
+++ b/net/ipv4/ah4.c
@@ -182,7 +182,8 @@
 	}
 	((struct iphdr*)work_buf)->protocol = ah->nexthdr;
 	skb->nh.raw += ah_hlen;
-	skb->h.raw = memcpy(skb_network_header(skb), work_buf, ihl);
+	memcpy(skb_network_header(skb), work_buf, ihl);
+	skb->h.raw = skb->nh.raw;
 	__skb_pull(skb, ah_hlen + ihl);
 
 	return 0;