random: Fix behavior of kernel option "-rs".

[pintos-anon] / src / tests / filesys / extended / Rubric.robustness

Robustness of file system:
1       dir-empty-name
1       dir-open
1       dir-over-file
1       dir-under-file

3       dir-rm-cwd
2       dir-rm-parent
1       dir-rm-root