if_geneve: Add Support for Geneve (RFC8926)

geneve creates a generic network virtualization tunnel interface
for Tentant Systems over an L3 (IP/UDP) underlay network that provides
a Layer 2 (ethernet) or Layer 3 service using the geneve protocol.
This implementation is based on RFC8926.

Reviewed by: glebius, adrian
Discussed with: zlei, kp
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D54172