staging: lustre: mgc: fix 'error handling' issues

Fix 'error handling' issues found by Coverity version 6.6.1:
Unchecked return value (CHECKED_RETURN)
Calling function without checking return value.
Argument cannot be negative (NEGATIVE_RETURNS)
Negative value used as argument to a function expecting a
positive value.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4055
Reviewed-on: http://review.whamcloud.com/7842


Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>