[SK_BUFF]: Introduce skb_network_header()

For the places where we need a pointer to the network header, it is still legal
to touch skb->nh.raw directly if just adding to, subtracting from or setting it
to another layer header.

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

include/linux/skbuff.h

@@ -960,6 +960,11 @@ static inline void skb_reserve(struct sk_buff *skb, int len)
 	skb->tail += len;
 }
 
+static inline unsigned char *skb_network_header(const struct sk_buff *skb)
+{
+	return skb->nh.raw;
+}
+
 static inline void skb_reset_network_header(struct sk_buff *skb)
 {
 	skb->nh.raw = skb->data;