lib/string_choices: Add str_plural() helper

Add str_plural() helper to replace existing open implementations
used by many drivers and help improve future user facing messages.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://lore.kernel.org/r/20240214165015.1656-1-michal.wajdeczko@intel.com


Signed-off-by: Kees Cook <keescook@chromium.org>