hyperv/hn: Add HN_DEBUG kernel option.

If bufring is used for per-TX ring descs, don't update "available"
counter, which is only used to help debugging.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8674

hyperv/hn: Don't hold txdesc, if no BPFs are attached.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8675