Add 2 interfaces to stat io throttle information:
  blkio.throttle.total_io_queued
  blkio.throttle.total_bytes_queued

These interfaces are used for monitoring throttled io/bytes and
analyzing if delay has relation with io throttle.
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NGavin Shan <shan.gavin@linux.alibaba.com>
Reviewed-by: NJiufei Xue <jiufei.xue@linux.alibaba.com>
Reviewed-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>