[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.

core/src/main/scala/org/apache/spark/storage/DiskBlockObjectWriter.scala

@@ -95,6 +95,7 @@ private[spark] class DiskBlockObjectWriter(
   /**
    * Keep track of number of records written and also use this to periodically
    * output bytes written since the latter is expensive to do for each record.
+   * And we reset it after every commitAndGet called.
    */
   private var numRecordsWritten = 0
 
@@ -185,6 +186,7 @@ private[spark] class DiskBlockObjectWriter(
       // In certain compression codecs, more bytes are written after streams are closed
       writeMetrics.incBytesWritten(committedPosition - reportedPosition)
       reportedPosition = committedPosition
+      numRecordsWritten = 0
       fileSegment
     } else {
       new FileSegment(file, committedPosition, 0)

core/src/test/scala/org/apache/spark/storage/DiskBlockObjectWriterSuite.scala

@@ -116,6 +116,7 @@ class DiskBlockObjectWriterSuite extends SparkFunSuite with BeforeAndAfterEach {
     writer.revertPartialWritesAndClose()
     assert(firstSegment.length === file.length())
     assert(writeMetrics.bytesWritten === file.length())
+    assert(writeMetrics.recordsWritten == 1)
   }
 
   test("calling revertPartialWritesAndClose() after commit() should have no effect") {