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>