hyperv/hn: Pass RX packet info to netvsc_recv.

This paves to nuke netvsc_packet, which does not serves much
purpose now.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7541

hyperv/hn: Constify RNDIS messages on RX path.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7542

hyperv/hn: Get rid of the useless netvsc_packet

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7544