hyperv/hn: Change global tunable prefix to hw.hn

And use SYSCTL+CTLFLAG_RDTUN for them.

Suggested by:       adrian
Reviewed by:        adrian, Hongjiang Zhang <honzhan microsoft com>
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5274

hyperv/hn: Split RX ring data structure out of softc

This paves the way for upcoming vRSS stuffs and eases more code cleanup.

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5275

hyperv/hn: Use taskqueue_enqueue()

This also eases experiment on the non-fast taskqueue.

Reviewed by:        adrian, Jun Su <junsu microsoft com>
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5276

hyperv/hn: Use non-fast taskqueue for transmission

Performance stays same; so no need to use fast taskqueue here.

hyperv/hn: Split TX ring data structure out of softc

This paves the way for upcoming vRSS stuffs and eases more code cleanup.

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5283