security: tomoyo: Fix obsolete function

simple_strtoul is obsolete, and use kstrtouint instead

Signed-off-by: Ding Xiang <dingxiang@cmss.chinamobile.com>
Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <james.morris@microsoft.com>