TOMOYO: Fix race on updating profile's comment line.

In tomoyo_write_profile() since 2.6.34, a lock was by error missing when
replacing profile's comment line. If multiple threads attempted

  echo '0-COMMENT=comment' > /sys/kernel/security/tomoyo/profile

in parallel, garbage collector will fail to kfree() the old value.
Protect the replacement using a lock. Also, keep the old value rather than
replace with empty string when out of memory error has occurred.

Signed-off-by: Xiaochen Wang <wangxiaochen0@gmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>