hyperv/hn: Use channel0, i.e. TX ring0, for TCP SYN/SYN|ACK.

Hyper-V hot channel effect:
Operation latency on hot channel is only _half_ of the operation
latency on cold channels.

This commit takes the advantage of the above Hyper-V host channel
effect, and can reduce more than 75% latency and more than 50%
latency stdev, i.e. lower and more stable/predictable latency,
for various types of web server workloads.

MFC after: 3 days
Sponsored by: Microsoft