Fix the following warning:

  usr/include/linux/string.h:8: userspace cannot reference function or variable defined in the kernel
Signed-off-by: NWANG Cong <xiyou.wangcong@gmail.com>
Acked-by: NAkinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>