stackleak: Use str_enabled_disabled() helper in stack_erasing_sysctl()

Remove hard-coded strings by using the str_enabled_disabled() helper
function.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20241222223157.135164-2-thorsten.blum@linux.dev


Signed-off-by: Kees Cook <kees@kernel.org>