Fix unchecked warnings in main Hadoop code under Java 6.

Reported by Tahir Hashmi:

I get the following warning while building:

[javac]
/home/tahir/Desktop/Trunk/trunk/src/test/org/apache/hadoop/dfs/ClusterTestDFS.java:439:
warning: [unchecked] unchecked call to
getConstructor(java.lang.Class<?>...) as a member of the raw type
java.lang.Class
[javac] randomDataGeneratorCtor = clazz.getConstructor(new
Class[]

);
[javac] ^
[javac] Note: Some input files use or override a deprecated API.