MFC 317353

MFC 317353

    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.

    Sponsored by:   Microsoft

git-svn-id: svn://svn.freebsd.org/base/stable/10@317482 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f