TOMOYO: Fix incorrect enforce mode.

In tomoyo_get_mode() since 2.6.36, CONFIG::file::execute was by error used in
place of CONFIG::file if CONFIG::file::execute was set to other than default.
As a result, enforcing mode was not applied in a way documentation says.

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