【OLK-5.10】io hung住时 iostat 平均队列长度显示为0

问题根因：

iostat计算平均队列长度时读取的是diskstats的nsecs字段，更新接口为：

part_stat_add(rq, nsecs[], xxx);

该接口目前只有在io完成时才更新，因此io hung住时无法更新

修复方案：

在iostat触发读取该字段时，主动去更新

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