[PKTGEN]: Centralize packet overhead tracking

Track the extra packet overhead for VLAN tags, MPLS, IPSEC etc

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>