tomoyo: avoid get+put of task_struct

Use task_cred_xxx(task, security) in tomoyo_real_domain() to
avoid a get+put of the target cred.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>