virtio: correct the memory barrier in virtqueue_kick_prepare()

Use virtio_mb() to make sure the available index to be exposed before
checking the the avail event. Otherwise we may get stale value of
avail event in guest and never kick the host after.

Note: this fixes a bug introduced by ee7cd89.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org

drivers/virtio/virtio_ring.c

@@ -308,9 +308,9 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
 	bool needs_kick;
 
 	START_USE(vq);
-	/* Descriptors and available array need to be set before we expose the
-	 * new available array entries. */
-	virtio_wmb(vq);
+	/* We need to expose available array entries before checking avail
+	 * event. */
+	virtio_mb(vq);
 
 	old = vq->vring.avail->idx - vq->num_added;
 	new = vq->vring.avail->idx;