Get more accurate space info for DedicatedDiskSpaceUsage

Java has 2 apis to get available space info for a mounted fs: `getFreeSpace()` and `getUsableSpace()`,  the latter is more accurate(see its java doc).

Let's do a experiment:

// java

import java.io.File;

public class FsSpace {
  public static void main(String args[]) {
    File file = new File(args[0]);
    System.out.println("FreeSpace: " + file.getFreeSpace());
    System.out.println("UsableSpace: " + file.getUsableSpace());
  }
}

 compile it and run against my root /, let's compare with linux command 'df'.

// linux command line
$ java FsSpace /
FreeSpace: 49653022720           <-- A
UsableSpace: 45126942720         <-- B

$ df / -B1
Filesystem        1B-blocks        Used   Available Use% Mounted on
overlay        105553784832 55900872704 45126832128  56% /
                                             ^
                                             |
                                             C

So the B and C is more close to each other, while A is larger.

Here we have about a 4GB diff on a 100GB disk for the 2 java apis.

I think that `getFreeSpace()` must only account for all unallocated space but does not take the space reserved for system use into consideration while `getUsableSpace()` do.

So we should be more conservative to prevent over consumed disk space.

P.S.

HDFS has a implemtation that use the 2 apis for different purposes, see:

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/DF.java