selftests/landlock: Fix readlink check

selftests/landlock: Fix readlink check

The audit_init_filter_exe() helper incorrectly checks the readlink(2)
error because an unsigned integer is used to store the result.  Use a
signed integer for this check.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/aDbFwyZ_fM-IO7sC@stanley.mountain
Fixes: 6a500b22971c ("selftests/landlock: Add tests for audit flags and domain IDs")
Reviewed-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250528144426.1709063-1-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>