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

sys/dev/hyperv/vmbus/hv_vmbus_drv_freebsd.c

@@ -117,7 +117,7 @@ handled:
 	     * not deliver any more messages
 	     * since there is no empty slot
 	     */
-	    wmb();
+	    atomic_thread_fence_seq_cst();
 
 	    if (msg->header.message_flags.u.message_pending) {
 			/*
@@ -187,7 +187,7 @@ hv_vmbus_isr(struct trapframe *frame)
 		 * not deliver any more messages
 		 * since there is no empty slot
 		 */
-		wmb();
+		atomic_thread_fence_seq_cst();
 
 		if (msg->header.message_flags.u.message_pending) {
 			/*