virtio/ringtest: virtio_ring: 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.
Reported-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>

tools/virtio/ringtest/virtio_ring_0_9.c

@@ -225,15 +225,17 @@ bool enable_call()
 
 void kick_available(void)
 {
+	bool need;
+
 	/* Flush in previous flags write */
 	/* Barrier C (for pairing) */
 	smp_mb();
-	if (!vring_need_event(vring_avail_event(&ring),
+	need = vring_need_event(vring_avail_event(&ring),
 				guest.avail_idx,
-			      guest.kicked_avail_idx))
-		return;
+				guest.kicked_avail_idx);
 
 	guest.kicked_avail_idx = guest.avail_idx;
+	if (need)
 		kick();
 }
 
@@ -316,14 +318,16 @@ 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 (!vring_need_event(vring_used_event(&ring),
+	need = vring_need_event(vring_used_event(&ring),
 				host.used_idx,
-			      host.called_used_idx))
-		return;
+				host.called_used_idx);
 
 	host.called_used_idx = host.used_idx;
+	if (need)
 		call();
 }