tcm_vhost: Use ACCESS_ONCE for vs->vs_tpg[target] access

In vhost_scsi_handle_vq:

      tv_tpg = vs->vs_tpg[target];
      if (!tv_tpg) {
              ....
              return
      }

      tv_cmd = vhost_scsi_allocate_cmd(tv_tpg, &v_req,

1) vs->vs_tpg[target] might change after the NULL check and 2) the above
line might access tv_tpg from vs->vs_tpg[target]. To prevent 2), use
ACCESS_ONCE. Thanks mst for catching this up!
Signed-off-by: Asias He <asias@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/vhost/tcm_vhost.c

@@ -661,7 +661,7 @@ static void vhost_scsi_handle_vq(struct vhost_scsi *vs,
 
 		/* Extract the tpgt */
 		target = v_req.lun[1];
-		tv_tpg = vs->vs_tpg[target];
+		tv_tpg = ACCESS_ONCE(vs->vs_tpg[target]);
 
 		/* Target does not exist, fail the request */
 		if (unlikely(!tv_tpg)) {