Epsilon: Premature OOM when allocating object larger than uncommitted heap size

When an object size meets the following three conditions, the allocation will fail unexpectedly.

1. `object_size` > `uncommitted_size`
2. `object_size` > `committed_but_unused_size`
3. `object_size` < `unused_size` (`unused_size` equals `uncommitted_size + committed_but_unused_size`)


The corresponding test case is shown below.

```
package gc.epsilon;
/**
 * @test TestEnoughUnusedSpace
 * @requires vm.gc.Epsilon
 * @summary Epsilon should allocates object successfully if it has enough space.
 * @run main/othervm -XX:InitialHeapSize=64M -Xmx128M -XX:+UnlockExperimentalVMOptions
 * -XX:+UseEpsilonGC gc.epsilon.TestEnoughUnusedSpace
 */

public class TestEnoughUnusedSpace {
    public static void main(String[] args) {
        byte[] arr = new byte[90 * 1024 * 1024];
        System.out.println(arr.length);
    }
}
```