hyperv/hn: Allow enabling IPv6 TX checksum offloading and IPv6 TSO.

They are still disabled by default.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8490

hyperv/hn: Don't abuse hn_{tx,rx}_ring_inuse.

Just in case, the # of TX/RX rings is changed upon synthetic parts
re-attach.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8520