LU-1812 mgs: assign PTR_ERR() to prevent warnings

Some newer kernels fail with the "error: ignoring return value of
'PTR_ERR', declared with attribute warn_unused_result".

Signed-off-by: chas williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Change-Id: I94ff7d8371e2daf7440dd5fb295c209ed671bc74
Reviewed-on: http://review.whamcloud.com/4971
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Peng Tao <bergwolf@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>