mount/fuse: invalidate entry upon setattr

It might need to be revalidated against storage backend if mode is changed,
since when ACLs are in use they might grant extra access with group part of the
mode as a mask.

Added a test.

Change-Id: Ib81a31b68c015662bc93a6ae4b9b51aa2302da55
BUG: 998967
Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com>