lib/vsprintf: do not show no_hash_pointers message multiple times

Do not show no_hash_pointers message multiple times if the option was
passed more than once (e.g. via generated command line).

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210305194206.3165917-1-elver@google.com