MFC 323728,323729

MFC 323728,323729

323728
    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

323729
    hyperv/hn: Incease max supported MTU

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@324466 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f