Fix statvfs filesystem - it is unstable in docker environments

[matthiasblaesing]

Instead of iterating mountpoints only query the information of the root directory and use NIO to query the java comparison data.

[matthiasblaesing]

@dbwiddis could you have a look at this? On the google mailinglist was reported, that the unittest for LibC#statvfs fails in a docker environment.

The issue is, that in a docker environment the mounts of the host system are reported, but the device files are not in place. So instead of checking random directories the change just checks the root file system. For that filesystem information must be present. Combined with java NIO we can verify the values. What do you think?

[dbwiddis]

No argument switching to NIO but shouldn't we be checking the subdirectories of /dev? If you only look at the root directory I don't think you're actually testing any filesystems.

[dbwiddis]

Actually, I think this is good. Looking at the docs we just need "any file within the mounted filesystem" so in this case we are assuming / is always mounted and just testing that one device.

[matthiasblaesing]

@dbwiddis thanks for checking. Yes the quoted description was my reasoning and I would be a bit irritated if you could run java programms without having a filesystem mounted (yes I can imaging a unikernel, but then you are far away from unittest land).