[SPARK-8476] [CORE] Setters inc/decDiskBytesSpilled in TaskMetrics should also be private.

This is a follow-up of [SPARK-3288](https://issues.apache.org/jira/browse/SPARK-3288).

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #6896 from ueshin/issues/SPARK-8476 and squashes the following commits:

89251d8 [Takuya UESHIN] Make inc/decDiskBytesSpilled in TaskMetrics private[spark].

core/src/main/scala/org/apache/spark/executor/TaskMetrics.scala

@@ -94,8 +94,8 @@ class TaskMetrics extends Serializable {
    */
   private var _diskBytesSpilled: Long = _
   def diskBytesSpilled: Long = _diskBytesSpilled
-  def incDiskBytesSpilled(value: Long): Unit = _diskBytesSpilled += value
-  def decDiskBytesSpilled(value: Long): Unit = _diskBytesSpilled -= value
+  private[spark] def incDiskBytesSpilled(value: Long): Unit = _diskBytesSpilled += value
+  private[spark] def decDiskBytesSpilled(value: Long): Unit = _diskBytesSpilled -= value
 
   /**
    * If this task reads from a HadoopRDD or from persisted data, metrics on how much data was read