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>