MFC 297635

MFC 297635

    hyperv/vmbus: Use default mtx for channel message queue

    First of all sema_post() can't be called w/ spinlock, and the channel
    message queue processing is not on hot code path, i.e. spinlock is not
    necessary.

    Submitted by:       Jun Su <junsu microsoft com>
    Reviewed by:        sephe, Dexuan Cui <decui microsoft com>
    MFC after:  1 week
    Sponsored by:       Microsoft OSTC
    Differential Revision:      https://reviews.freebsd.org/D5812

git-svn-id: svn://svn.freebsd.org/base/stable/10@301925 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f