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

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