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.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D12352

hyperv/hn: Incease max supported MTU

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D12365