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