MFC 296180,297634

MFC 296180,297634

296180
    hyperv: Use proper fence function to keep store-load order for msgs

    sfence only makes sure about the store-store order, which is not
    sufficient here.  Use atomic_thread_fence_seq_cst() as suggested
    jhb and kib (a locked op in the nutshell, which should have the

    Reviewed by:        jhb, kib, Jun Su <junsu microsoft com>
    MFC after:  1 week
    Sponsored by:       Microsoft OSTC
    Differential Revision:      https://reviews.freebsd.org/D5436

297634
    hyperv: Use mb() instead of atomic_thread_fence_seq_cst()

    Since atomic_thread_fence_seq_cst() will become compiler fence on UP kernel.

    Reviewed by:        kib, Dexuan Cui <decui microsoft com>
    MFC after:  1 week
    Sponsored by:       Microsoft OSTC
    Differential Revision:      https://reviews.freebsd.org/D5852

git-svn-id: svn://svn.freebsd.org/base/stable/10@301914 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f