[IPSEC]: Use IPv6 calling convention as the convention for x->mode->output

The IPv6 calling convention for x->mode->output is more general and could
help an eventual protocol-generic x->type->output implementation.  This
patch adopts it for IPv4 as well and modifies the IPv4 type output functions
accordingly.

It also rewrites the IPv6 mac/transport header calculation to be based off
the network header where practical.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ipcomp.c b/net/ipv4/ipcomp.c
index 1929d451d..bf74f64 100644
--- a/net/ipv4/ipcomp.c
+++ b/net/ipv4/ipcomp.c
@@ -98,10 +98,10 @@
 static int ipcomp_compress(struct xfrm_state *x, struct sk_buff *skb)
 {
 	struct ipcomp_data *ipcd = x->data;
-	const int ihlen = ip_hdrlen(skb);
+	const int ihlen = skb_transport_offset(skb);
 	const int plen = skb->len - ihlen;
 	int dlen = IPCOMP_SCRATCH_SIZE;
-	u8 *start = skb->data + ihlen;
+	u8 *start = skb_transport_header(skb);
 	const int cpu = get_cpu();
 	u8 *scratch = *per_cpu_ptr(ipcomp_scratches, cpu);
 	struct crypto_comp *tfm = *per_cpu_ptr(ipcd->tfms, cpu);
@@ -154,11 +154,11 @@
 
 	/* Install ipcomp header, convert into ipcomp datagram. */
 	iph->tot_len = htons(skb->len);
-	ipch = (struct ip_comp_hdr *)((char *)iph + iph->ihl * 4);
-	ipch->nexthdr = iph->protocol;
+	ipch = (struct ip_comp_hdr *)skb_transport_header(skb);
+	ipch->nexthdr = *skb_mac_header(skb);
 	ipch->flags = 0;
 	ipch->cpi = htons((u16 )ntohl(x->id.spi));
-	iph->protocol = IPPROTO_COMP;
+	*skb_mac_header(skb) = IPPROTO_COMP;
 	ip_send_check(iph);
 	return 0;