[SPARK-23358][CORE] When the number of partitions is greater than 2^2…

…8, it will result in an error result

## What changes were proposed in this pull request?
In the `checkIndexAndDataFile`,the `blocks` is the ` Int` type,  when it is greater than 2^28, `blocks*8` will overflow, and this will result in an error result.
In fact, `blocks` is actually the number of partitions.

## How was this patch tested?
Manual test

Author: liuxian <liu.xian3@zte.com.cn>

Closes apache#20544 from 10110346/overflow.

core/src/main/scala/org/apache/spark/shuffle/IndexShuffleBlockResolver.scala

@@ -84,7 +84,7 @@ private[spark] class IndexShuffleBlockResolver(
    */
   private def checkIndexAndDataFile(index: File, data: File, blocks: Int): Array[Long] = {
     // the index file should have `block + 1` longs as offset.
-    if (index.length() != (blocks + 1) * 8) {
+    if (index.length() != (blocks + 1) * 8L) {
       return null
     }
     val lengths = new Array[Long](blocks)