[SK_BUFF]: Introduce skb_network_header_len

For the common sequence "skb->h.raw - skb->nh.raw", similar to skb->mac_len,
that is precalculated tho, don't think we need to bloat skb with one more
member, so just use this new helper, reducing the number of non-skbuff.h
references to the layer headers even more.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/xfrm6_policy.c b/net/ipv6/xfrm6_policy.c
index b93bfb8..ef746d4 100644
--- a/net/ipv6/xfrm6_policy.c
+++ b/net/ipv6/xfrm6_policy.c
@@ -270,7 +270,7 @@
 static inline void
 _decode_session6(struct sk_buff *skb, struct flowi *fl)
 {
-	u16 offset = skb->h.raw - skb->nh.raw;
+	u16 offset = skb_network_header_len(skb);
 	struct ipv6hdr *hdr = ipv6_hdr(skb);
 	struct ipv6_opt_hdr *exthdr;
 	const unsigned char *nh = skb_network_header(skb);