[SPARK-21621][CORE] Reset numRecordsWritten after DiskBlockObjectWriter.commitAndGet called

## What changes were proposed in this pull request?

We should reset numRecordsWritten to zero after DiskBlockObjectWriter.commitAndGet called.
Because when `revertPartialWritesAndClose` be called, we decrease the written records in `ShuffleWriteMetrics` . However, we decreased the written records to zero, this should be wrong, we should only decreased the number reords after the last `commitAndGet` called.

## How was this patch tested?
Modified existing test.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Xianyang Liu <xianyang.liu@intel.com>

Closes #18830 from ConeyLiu/DiskBlockObjectWriter.