LU-3179 build: fix compilation error with gcc 4.7.2

use cast to resolve build warning from gcc 4.7.2

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I4356e621f5b9c687a6b76b12a93b664d542e1c79
Reviewed-on: http://review.whamcloud.com/6141
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Nikitas Angelinas <nikitas_angelinas@xyratex.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>