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