[SPARK-9923] [CORE] ShuffleMapStage.numAvailableOutputs should be an Int instead of Long

Modified type of ShuffleMapStage.numAvailableOutputs from Long to Int

Author: Neelesh Srinivas Salian <nsalian@cloudera.com>

Closes #8183 from nssalian/SPARK-9923.

core/src/main/scala/org/apache/spark/scheduler/ShuffleMapStage.scala

@@ -37,7 +37,7 @@ private[spark] class ShuffleMapStage(
 
   override def toString: String = "ShuffleMapStage " + id
 
-  var numAvailableOutputs: Long = 0
+  var numAvailableOutputs: Int = 0
 
   def isAvailable: Boolean = numAvailableOutputs == numPartitions