net/skbuff: Introduce skb_mac_offset()

Introduce skb_mac_offset() that could be used to get mac header offset.
Signed-off-by: Amir Vadai <amir@vadai.me>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -2184,6 +2184,11 @@ static inline unsigned char *skb_mac_header(const struct sk_buff *skb)
 	return skb->head + skb->mac_header;
 }
 
+static inline int skb_mac_offset(const struct sk_buff *skb)
+{
+	return skb_mac_header(skb) - skb->data;
+}
+
 static inline int skb_mac_header_was_set(const struct sk_buff *skb)
 {
 	return skb->mac_header != (typeof(skb->mac_header))~0U;