[SK_BUFF]: Introduce ipv6_hdr(), remove skb->nh.ipv6h

Now the skb->nh union has just one member, .raw, i.e. it is just like the
skb->mac union, strange, no? I'm just leaving it like that till the transport
layer is done with, when we'll rename skb->mac.raw to skb->mac_header (or
->mac_header_offset?), ditto for ->{h,nh}.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ipv6.h

@@ -223,6 +223,11 @@ enum {
 #include <net/if_inet6.h>       /* struct ipv6_mc_socklist */
 #include <net/inet_sock.h>
 
+static inline struct ipv6hdr *ipv6_hdr(const struct sk_buff *skb)
+{
+	return (struct ipv6hdr *)skb_network_header(skb);
+}
+
 /* 
    This structure contains results of exthdrs parsing
    as offsets from skb->nh.