hyperv/hn: Implement ifnet.if_transmit method

It will be turned on by default later.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5415

hyperv/hn: Hold the TX ring lock then drain TX desc buf_ring

Reported by:        Hongxiong Xian <v-hoxian microsoft com>
MFC after:  1 week
Sponsored by:       Microsoft OSTC

hyperv: Use atomic_fetchadd_int to get GPADL id.

Reviewed by:        Hongjiang Zhang <honzhan microsoft com>
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5439