string_choices: Add wrapper for str_down_up()

The string choice functions which are not clearly true/false synonyms
also have inverted wrappers. Add this for str_down_up() as well.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20240812182939.work.424-kees@kernel.org


Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Kees Cook <kees@kernel.org>