[IPSEC]: Set skb->data to payload in x->mode->output

This patch changes the calling convention so that on entry from
x->mode->output and before entry into x->type->output skb->data
will point to the payload instead of the IP header.

This is essentially a redistribution of skb_push/skb_pull calls
with the aim of minimising them on the common path of tunnel +
ESP.

It'll also let us use the same calling convention between IPv4
and IPv6 with the next patch.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/ipcomp6.c b/net/ipv6/ipcomp6.c
index 71a14c0..87e6407e 100644
--- a/net/ipv6/ipcomp6.c
+++ b/net/ipv6/ipcomp6.c
@@ -128,7 +128,10 @@
 	u8 *start, *scratch;
 	struct crypto_comp *tfm;
 	int cpu;
-	int hdr_len = skb_transport_offset(skb);
+	int hdr_len;
+
+	skb_push(skb, -skb_network_offset(skb));
+	hdr_len = skb_transport_offset(skb);
 
 	/* check whether datagram len is larger than threshold */
 	if ((skb->len - hdr_len) < ipcd->threshold) {