fix null pointer exception for filesystemperf

If the parent dir does not exist, the mkdir() will fail,
then File.list() will return null, which causes *NULL POINTER*
exception. Just change mkdir to mkdirs.

Change-Id: I52cc3262c111c32d24f62d05c02cb42687731a31
diff --git a/suite/cts/deviceTests/filesystemperf/src/com/android/cts/filesystemperf/FileUtil.java b/suite/cts/deviceTests/filesystemperf/src/com/android/cts/filesystemperf/FileUtil.java
index 6231774..4007e03 100755
--- a/suite/cts/deviceTests/filesystemperf/src/com/android/cts/filesystemperf/FileUtil.java
+++ b/suite/cts/deviceTests/filesystemperf/src/com/android/cts/filesystemperf/FileUtil.java
@@ -73,7 +73,7 @@
      */
     public static File createNewFile(Context context, String dirName) {
         File topDir = new File(context.getFilesDir(), dirName);
-        topDir.mkdir();
+        topDir.mkdirs();
         String[] list = topDir.list();
 
         String newFileName;