LU-839 tests: test_52 fails with loop devices MRP-233

LU-839 tests: test_52 fails with loop devices MRP-233

During loop mount actual device is replaced with /dev/loopX
Use mount path as umount parameter.

Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Change-Id: Ic0561ccfd52036d7ed18e05848be7b4342f4cc62
Reviewed-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-by: Bruce Korb <bruce_korb@xyratex.com>
Reviewed-on: http://review.whamcloud.com/1694
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>