du: reuse file name buf

[oech3]

du_deep_tree[(100, 3)] +1%
du_all_wide_tree[(5000, 500)] +1%
du_max_depth_balanced_tree[(6, 4, 10)] 25.7 -> 25.6 ms

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/resolution. tests/date/resolution is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/misc/io-errors. tests/misc/io-errors is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/timeout/timeout-group. tests/timeout/timeout-group is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/tail/retry is no longer failing!
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!

[sylvestre]

could you please handle the current draft before opening new PR? thanks :)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Congrats! The gnu test tests/basenc/bounded-memory is now passing!
Congrats! The gnu test tests/dd/no-allocate is now passing!