TOMOYO: Allow reading only execute permission.

Policy editor needs to know allow_execute entries in order to build domain
transition tree. Reading all entries is slow. Thus, allow reading only
allow_execute entries.

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