hyperv/hn: Use buf_ring for txdesc list

So one spinlock is avoided, which would be potentially dangerous for
virtual machine, if the spinlock holder was scheduled out by the host,
as noted by royger.

Old spinlock based txdesc list is still kept around, so we could have
a safe fallback.

No performance regression nor improvement is observed.

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5290

hyperv/hn: Add option to bind TX taskqueues to the specified CPU

It will be used to help tracking host side transmission ring selection
issue; and it will be turned on by default, once we have concrete result.

Reviewed by:        adrian, Jun Su <junsu microsoft com>
Approved by:        adrian (mento)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5316

hyperv/hn: Enable IP header checksum offloading for WIN8 (WinServ2012)

Tested on Windows Server 2012.

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5317

hyperv/hn: Free the txdesc buf_ring when the TX ring is destroyed

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5318