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

hyperv/hn: Reorganize the synthetic parts detach.

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

hyperv/hn: Reorder the comment a little bit.

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