MFC 306390-306392

MFC 306390-306392

306390
    hyperv/hn: Suspend and resume the backend properly upon MTU change.

    Suspend:
    - Prevent the backend from being touched on TX path.
    - Clear the RNDIS RX filter, and wait for RX to drain.
    - Make sure that NVS see the chimney sending buffer and RXBUF
      disconnection, before unlink these buffers from the channel.

    Resume:
    - Reconfigure the RNDIS filter.
    - Allow TX path to work on the backend.
    - Kick start the TX eof task, in case the OACTIVE is set.

    This fixes various panics, when the interface has traffic and MTU
    is being changed.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D8046

306391
    hyperv/hn: Reorganize the synthetic parts detach.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D8047

306392
    hyperv/hn: Reorder the comment a little bit.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D8048

git-svn-id: svn://svn.freebsd.org/base/stable/10@307250 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f