net: Add skb_inner_transport_offset function

Same thing as skb_transport_offset but returns the offset of the inner
transport header (when skb->encpasulation is set).
Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1939,6 +1939,11 @@ static inline unsigned char *skb_inner_transport_header(const struct sk_buff
 	return skb->head + skb->inner_transport_header;
 }
 
+static inline int skb_inner_transport_offset(const struct sk_buff *skb)
+{
+	return skb_inner_transport_header(skb) - skb->data;
+}
+
 static inline void skb_reset_inner_transport_header(struct sk_buff *skb)
 {
 	skb->inner_transport_header = skb->data - skb->head;