hyperv/hn: Increase LRO entry count to 128 by default

hn(4) only has one RX ring currently, so default 8 LRO entries
are too small.

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5166

hyperv/hn: Move LRO flush to the channel processing rollup

This significantly increases LRO aggregation ratio when there are
large amount of connections (improves reception performance a lot).

Reviewed by:        adrian
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5167

hyperv/hn: Add an option to always do transmission scheduling

It is off by default. This eases more experiment on hn(4).

Reviewed by:        adrian, Hongjiang Zhang <honzhan microsoft com>
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5175