hyperv/hn: Fix UDP checksum offload issue in Azure.

UDP checksum offload does not work in Azure if following conditions are
met:
- sizeof(IP hdr + UDP hdr + payload) > 1420.
- IP_DF is not set in IP hdr

Use software checksum for UDP datagrams falling into this category.

Add two tunables to disable UDP/IPv4 and UDP/IPv6 checksum offload, in
case something unexpected happened.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D12429

hyperv/hn: Unbreak i386 building.

Reported by:    cy
Sponsored by:   Microsoft