selftests/landlock: Add filesystem access benchmark

selftests/landlock: Add filesystem access benchmark

fs_bench benchmarks the performance of Landlock's path walk
by exercising it in a scenario that amplifies Landlock's overhead:

* Create a large number of nested directories
* Enforce a Landlock policy in which a rule is associated with each of
  these subdirectories
* Benchmark openat() applied to the deepest directory,
  forcing Landlock to walk the entire path.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260206151154.97915-3-gnoack3000@gmail.com
[mic: Fix missing mode with O_CREAT, improve text consistency, sort
includes]
Signed-off-by: Mickaël Salaün <mic@digikod.net>