[IPSEC]: Get rid of ipv6_{auth,esp,comp}_hdr

This patch removes the duplicate ipv6_{auth,esp,comp}_hdr structures since
they're identical to the IPv4 versions.  Duplicating them would only create
problems for ourselves later when we need to add things like extended
sequence numbers.

I've also added transport header type conversion headers for these types
which are now used by the transforms.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/esp.h b/include/net/esp.h
index e793d769..c1bc529 100644
--- a/include/net/esp.h
+++ b/include/net/esp.h
@@ -53,4 +53,11 @@
 	return crypto_hash_final(&desc, esp->auth.work_icv);
 }
 
+struct ip_esp_hdr;
+
+static inline struct ip_esp_hdr *ip_esp_hdr(const struct sk_buff *skb)
+{
+	return (struct ip_esp_hdr *)skb_transport_header(skb);
+}
+
 #endif