Merge pull request apache#589 from mengxr/index.

SPARK-1076: Convert Int to Long to avoid overflow

Patch for PR apache#578.

Author: Xiangrui Meng <meng@databricks.com>

Closes apache#589 and squashes the following commits:

98c435e [Xiangrui Meng] cast Int to Long to avoid Int overflow

Author: mengxr

core/src/main/scala/org/apache/spark/rdd/RDD.scala

@@ -873,7 +873,7 @@ abstract class RDD[T: ClassTag](
    * won't trigger a spark job, which is different from [[org.apache.spark.rdd.RDD#zipWithIndex]].
    */
   def zipWithUniqueId(): RDD[(T, Long)] = {
-    val n = this.partitions.size
+    val n = this.partitions.size.toLong
     this.mapPartitionsWithIndex { case (k, iter) =>
       iter.zipWithIndex.map { case (item, i) =>
         (item, i * n + k)