du: Reduce malloc

[oech3]

No description provided.

[oech3]

du_summarize_balanced_tree[(5, 4, 10)] +2%
du_wide_tree[(5000, 500)] +1%
du_max_depth_balanced_tree[(6, 4, 10)] +1%
du_all_wide_tree[(5000, 500)] +1%

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/resolution. tests/date/resolution is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)