hyperv/hn: Properly handle synthetic parts reattach failure.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8310

hyperv/hn: Start link status check, if no network changes were pending.

Link status check is much more lightweight than network change detection.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8311