[SPARK-21033][CORE][FOLLOW-UP] Update Spillable

## What changes were proposed in this pull request?

Update
```scala
SparkEnv.get.conf.getLong("spark.shuffle.spill.numElementsForceSpillThreshold", Long.MaxValue)
```
to
```scala
SparkEnv.get.conf.get(SHUFFLE_SPILL_NUM_ELEMENTS_FORCE_SPILL_THRESHOLD)
```

 because of `SHUFFLE_SPILL_NUM_ELEMENTS_FORCE_SPILL_THRESHOLD`'s default value is `Integer.MAX_VALUE`:
https://github.com/apache/spark/blob/c99fc9ad9b600095baba003053dbf84304ca392b/core/src/main/scala/org/apache/spark/internal/config/package.scala#L503-L511

## How was this patch tested?

N/A

Author: Yuming Wang <yumwang@ebay.com>

Closes #21077 from wangyum/SPARK-21033.

Author: wangyum

core/src/main/scala/org/apache/spark/util/collection/Spillable.scala

@@ -19,6 +19,7 @@ package org.apache.spark.util.collection
 
 import org.apache.spark.SparkEnv
 import org.apache.spark.internal.Logging
+import org.apache.spark.internal.config._
 import org.apache.spark.memory.{MemoryConsumer, MemoryMode, TaskMemoryManager}
 
 /**
@@ -41,7 +42,7 @@ private[spark] abstract class Spillable[C](taskMemoryManager: TaskMemoryManager)
   protected def forceSpill(): Boolean
 
   // Number of elements read from input since last spill
-  protected def elementsRead: Long = _elementsRead
+  protected def elementsRead: Int = _elementsRead
 
   // Called by subclasses every time a record is read
   // It's used for checking spilling frequency
@@ -54,15 +55,15 @@ private[spark] abstract class Spillable[C](taskMemoryManager: TaskMemoryManager)
 
   // Force this collection to spill when there are this many elements in memory
   // For testing only
-  private[this] val numElementsForceSpillThreshold: Long =
-    SparkEnv.get.conf.getLong("spark.shuffle.spill.numElementsForceSpillThreshold", Long.MaxValue)
+  private[this] val numElementsForceSpillThreshold: Int =
+    SparkEnv.get.conf.get(SHUFFLE_SPILL_NUM_ELEMENTS_FORCE_SPILL_THRESHOLD)
 
   // Threshold for this collection's size in bytes before we start tracking its memory usage
   // To avoid a large number of small spills, initialize this to a value orders of magnitude > 0
   @volatile private[this] var myMemoryThreshold = initialMemoryThreshold
 
   // Number of elements read from input since last spill
-  private[this] var _elementsRead = 0L
+  private[this] var _elementsRead = 0
 
   // Number of bytes spilled in total
   @volatile private[this] var _memoryBytesSpilled = 0L