tomoyo: add missing rcu_dereference()

Adds a missed rcu_dereference() around real_parent.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>