hyperv/hn: Remove unnecessary zeroing out the netvsc_packet

All used fields are setup one by one, so there is no need to zero
out this large struct.

While I'm here, move the stack variable near its usage.

Reviewed by: adrian, delphij, Jun Su <junsu microsoft com>
Approved by: adrian (mentor)
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D4978