Call correct stop().

Oopsie in #504.

Author: Aaron Davidson <aaron@databricks.com>

Closes #527 from aarondav/stop and squashes the following commits:

8d1446a [Aaron Davidson] Call correct stop().

core/src/main/scala/org/apache/spark/storage/DiskBlockManager.scala

@@ -150,11 +150,12 @@ private[spark] class DiskBlockManager(shuffleManager: ShuffleBlockManager, rootD
     Runtime.getRuntime.addShutdownHook(new Thread("delete Spark local dirs") {
       override def run() {
         logDebug("Shutdown hook called")
-        stop()
+        DiskBlockManager.this.stop()
       }
     })
   }
 
+  /** Cleanup local dirs and stop shuffle sender. */
   private[spark] def stop() {
     localDirs.foreach { localDir =>
       if (localDir.isDirectory() && localDir.exists()) {