Applied patch for HDFS-4451 directly to our fork.

Tested like so:

$ mvn clean install -fn -Pnative -Dmaven.javadoc.skip=true -DskipTests -DHadoopPatchProcess
$ cd hadoop-hdfs-project/hadoop-hdfs
$ mvn test -fn -Drequire.test.libhadoop -DHadoopPatchProcess -Dtest=TestBalancer
...
-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running org.apache.hadoop.hdfs.server.balancer.TestBalancer
Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 61.528 sec

Results :

Tests run: 5, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1:17.148s
[INFO] Finished at: Tue Jan 29 14:08:06 PST 2013
[INFO] Final Memory: 26M/361M
[INFO] ------------------------------------------------------------------------