[SPARK-17417][CORE] Fix # of partitions for Reliable RDD checkpointing

## What changes were proposed in this pull request?
Currently the no. of partition files are limited to 10000 files (%05d format). If there are more than 10000 part files, the logic goes for a toss while recreating the RDD as it sorts them by string. More details can be found in the JIRA desc [here](https://issues.apache.org/jira/browse/SPARK-17417).

## How was this patch tested?
I tested this patch by checkpointing a RDD and then manually renaming part files to the old format and tried to access the RDD. It was successfully created from the old format. Also verified loading a sample parquet file and saving it as multiple formats - CSV, JSON, Text, Parquet, ORC and read them successfully back from the saved files. I couldn't launch the unit test from my local box, so will wait for the Jenkins output.

Author: Dhruve Ashar <dhruveashar@gmail.com>

Closes #15370 from dhruve/bug/SPARK-17417.

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

@@ -69,10 +69,10 @@ private[spark] class ReliableCheckpointRDD[T: ClassTag](
     val inputFiles = fs.listStatus(cpath)
       .map(_.getPath)
       .filter(_.getName.startsWith("part-"))
-      .sortBy(_.toString)
+      .sortBy(_.getName.stripPrefix("part-").toInt)
     // Fail fast if input files are invalid
     inputFiles.zipWithIndex.foreach { case (path, i) =>
-      if (!path.toString.endsWith(ReliableCheckpointRDD.checkpointFileName(i))) {
+      if (path.getName != ReliableCheckpointRDD.checkpointFileName(i)) {
         throw new SparkException(s"Invalid checkpoint file: $path")
       }
     }