fs: Fix fs_opendir resource leak when invoked on fs_dir_t object in use

Fixes problem when fs_opendir invoked on fs_dir_t object,
which is already holding information on opened directory,
overwrites references to other memory objects within
the fs_dir_t object causing resource leak.
If fs_opendir is invoked on already used fs_dir_t object,
it will return
-EBUSY.

Note: The change requires that all fs_dir_t objects should be
initialized to 0.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>