apparmor: Adjust offset when accessing task blob.

AppArmor will no longer be the only user of task blob
after TOMOYO started using task blob.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: f4ad8f2c

 ("LSM: Infrastructure management of the task security")
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>