virtio-net: define GSO over UDP tunnel offload

[pabeni]

UDP tunnels are ubicuos in many virtual network deployment, and currently both the virtio-net device and driver require segmentation of GSO over UDP tunnel traffic, impacting negatively all the workloads in such deployments.

The proposed solution is to define support for GSO over UDP tunnel in virtio-net.

spec patch: https://lore.kernel.org/virtio-comment/[email protected]/T/#mc9fb6e78200831e7fc2475f74e702c4c5d1eff50

[mstsirkin]

BALLOT CREATED AT URL: https://groups.oasis-open.org/higherlogic/ws/public/ballot?id=3868