hyperv/hn: Pass channel to hv_nv_on_receive_completion()

While I'm here, staticize this function.

Submitted by:       Hongjiang Zhang <honzhan microsoft com>
Modified by:        sephe
MFC after:  1 week
Sponsored by:       Microsoft OSTC

hyperv/hn: Make read buffer per-channel

Submitted by:       Hongjiang Zhang <honzhan microsoft com>
Reorganized by:     sephe
MFC after:  1 week
Sponsored by:       Microsoft OSTC

hyperv/hn: Fix typo in comment

MFC after:  1 week
Sponsored by:       Microsoft OSTC

hyperv/hn: Make # of rings configurable

And since the host may not being able to allocate the # of rings
requested by us, save the # of rings allocated by the host in the
ring_inuse counters; use ring_inuse counters for run time operation.

This paves the way for the upcoming vRSS support.

MFC after:  1 week
Sponsored by:       Microsoft OSTC