[SPARK-1527] change rootDir*.getName to rootDir*.getAbsolutePath

JIRA issue: [SPARK-1527](https://issues.apache.org/jira/browse/SPARK-1527)

getName() only gets the last component of the file path. When deleting test-generated directories,
we should pass the generated directory's absolute path to DiskBlockManager.

Author: Ye Xianjin <[email protected]>

This patch had conflicts when merged, resolved by
Committer: Patrick Wendell <[email protected]>

Closes #436 from advancedxy/SPARK-1527 and squashes the following commits:

4678bab [Ye Xianjin] change rootDir*.getname to rootDir*.getAbsolutePath so the temporary directories are deleted when the test is finished.

Author: advancedxy

core/src/test/scala/org/apache/spark/storage/DiskBlockManagerSuite.scala

@@ -52,7 +52,7 @@ class DiskBlockManagerSuite extends FunSuite with BeforeAndAfterEach with Before
     rootDir0.deleteOnExit()
     rootDir1 = Files.createTempDir()
     rootDir1.deleteOnExit()
-    rootDirs = rootDir0.getName + "," + rootDir1.getName
+    rootDirs = rootDir0.getAbsolutePath + "," + rootDir1.getAbsolutePath
     println("Created root dirs: " + rootDirs)
   }