Fix undefined reference issue reported by kbuild test robot.

Cc: <stable@vger.kernel.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>