vhost: Use vhost_get_used_size() in vhost_vring_set_addr()

The open-coded computation of the used size doesn't take the event
into account when the VIRTIO_RING_F_EVENT_IDX feature is present.
Fix that by using vhost_get_used_size().

Fixes: 8ea8cf8 ("vhost: support event index")
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kurz <groug@kaod.org>
Link: https://lore.kernel.org/r/160171932300.284610.11846106312938909461.stgit@bahia.lan
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -1519,8 +1519,7 @@ static long vhost_vring_set_addr(struct vhost_dev *d,
 		/* Also validate log access for used ring if enabled. */
 		if ((a.flags & (0x1 << VHOST_VRING_F_LOG)) &&
 			!log_access_ok(vq->log_base, a.log_guest_addr,
-				sizeof *vq->used +
-				vq->num * sizeof *vq->used->ring))
+				       vhost_get_used_size(vq, vq->num)))
 			return -EINVAL;
 	}