LU-4374 mgs: No modify during set_param is not an error

LU-4374 mgs: No modify during set_param is not an error

Non negative return code from mgs_modify(), and particularly
1 for no-modify because already set, is wrongly interpreted as
an error with associated msgs in debug log and at user-level.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Ieb058df406fd5e176cfe177a43dad0e6ebfbe78a
Reviewed-on: http://review.whamcloud.com/8567
Tested-by: Jenkins
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>