mkdir: do not ignore EEXIST on non-recursive creation

[l46983284-cpu]

Fixes #13970.

mkdir without -p incorrectly swallows EEXIST when the target path already exists as a directory. In create_single_dir, the error branch Err(_) if path.is_dir() returns Ok(()) unconditionally, without checking whether recursive (-p) mode was requested.

This is a logic bug: a non-recursive mkdir on an existing directory must fail per POSIX, but currently exits 0. Under concurrent invocation, this also allows multiple racing processes to all "succeed" in creating the same directory.

Fix: require config.recursive || is_parent before allowing the EEXIST → Ok(()) fallback. This matches GNU mkdir behavior and the existing test expectations.

Regression tests:

• test_mkdir_concurrent_creation: concurrent mkdir -p on the same path — all processes must succeed (valid for -p).
• test_mkdir_concurrent_non_recursive: concurrent mkdir without -p on the same path — exactly one process must win per round.

Verified on bare-metal Linux and in a sandboxed environment: 500 concurrent rounds on main showed double-wins (both processes exiting 0), while this branch showed 0 double-wins. All existing tests pass.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/empty-backup-suffix. tests/misc/empty-backup-suffix is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/misc/uname-labeled. tests/misc/uname-labeled is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)