hyperv/hn: Partly rework transmission path

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