hyperv/hn: Partly rework transmission path

• Avoid unnecessary malloc/free on transmission path.
• busdma(9)-fy transmission path.
• Properly handle IFF_DRV_OACTIVE. This should fix the network stalls reported by many.
• Properly setup TSO parameters.
• Properly handle bpf(4) tapping. This 5 times the performance during TCP sending test, when there is one bpf(4) attached.
• Allow size of chimney sending be tuned on a running system. Default value still needs more test to determine.

Reviewed by: adrian, delphij
Approved by: adrian (mentor)
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D4972