hyperv/hn: Fix MTU setting
- Add size of an ethernet header to the value configured to NVS. This does not seem to have any effects if MTU is 1500, but fix hypervisor side's setting if MTU > 1500.
- Override the MTU setting according to the view from the hypervisor side.
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D12352