Motivation:
As we don't explicitly create fs-root object, the fs
default value will be used as initial mode. An explicit
value will make tests more stable and don't rely on umask
value of the environment.

Modification:
enforce default file permission for DummyVFS

Result:
environment independent test behavior.

Acked-by: Paul Millar
Acked-by: Lea Morschel
Target: master