virtio/ringtest: fix up need_event math

last kicked event index must be updated unconditionally:
even if we don't need to kick, we do not want to re-check
the same entry for events.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

tools/virtio/ringtest/ring.c

@@ -188,15 +188,17 @@ bool enable_call()
 
 void kick_available(void)
 {
+	bool need;
+
 	/* Flush in previous flags write */
 	/* Barrier C (for pairing) */
 	smp_mb();
-	if (!need_event(event->kick_index,
+	need = need_event(event->kick_index,
 			   guest.avail_idx,
-			guest.kicked_avail_idx))
-		return;
+			   guest.kicked_avail_idx);
 
 	guest.kicked_avail_idx = guest.avail_idx;
+	if (need)
 		kick();
 }
 
@@ -253,14 +255,18 @@ bool use_buf(unsigned *lenp, void **bufp)
 
 void call_used(void)
 {
+	bool need;
+
 	/* Flush in previous flags write */
 	/* Barrier D (for pairing) */
 	smp_mb();
-	if (!need_event(event->call_index,
+
+	need = need_event(event->call_index,
 			host.used_idx,
-			host.called_used_idx))
-		return;
+			host.called_used_idx);
 
 	host.called_used_idx = host.used_idx;
+
+	if (need)
 		call();
 }