pcn-k8switch doesn't pass verification when using XDP_DRV mode

[acloudiator]

To reproduce:

polycubectl k8switch add k8switch1 loglevel=OFF \
cluster-ip-subnet=10.96.0.0/12 client-subnet=192.168.1.0/24 \
virtual-client-subnet=10.10.1.0/24 type=TYPE_XDP_DRV

The verifier will fails with:

math between pkt pointer and register with unbounded min value is not allowed

This is somehow related to the checksum update functions, but we still don't find the solution for it.

[mauriciovasquezbernal]

Could be related: https://github.com/iovisor/bcc/issues/2463