coccinelle: Add rules to find str_down_up() replacements

As done with str_up_down(), add checks for str_down_up() opportunities.
5 cases currently exist in the tree.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20240812183637.work.999-kees@kernel.org


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