TOMOYO: Remove memory pool for string data.

Currently, TOMOYO allocates memory for string data from memory pool allocated
by kmalloc(PAGE_SIZE). But that makes it difficult to kfree() when garbage
collector is added. Thus, remove memory pool and use kmalloc(strlen()).

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