Avoid redundant RPC calls for getDiskStatus

There are 3 RPC calls to fetch the same values :

  public FsStatus getDiskStatus() throws IOException {
    return new FsStatus(getStateByIndex(0),
        getStateByIndex(1), getStateByIndex(2));
  }

getStateByIndex() is called thrice, which is actually a getStats RPC to namenode, The same could have been achieved by just one call