du: some small refactorings

[cakebaker]

This PR applies three refactorings:

• it removes a condition where the if and else blocks are identical
• it removes the unused _options param from Stat::new_from_dirfd
• it simplifies two if/else constructs

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)