staging: lustre: fix 'error handling' issues for libcfs workitem.c

Fix 'error handling' issues found by Coverity version 6.5.1:
Unchecked return value (CHECKED_RETURN)
Calling function without checking return value.

Signed-off-by: Sebastien Buisson <sebastien.buisson@bull.net>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3427
Reviewed-on: http://review.whamcloud.com/7103


Reviewed-by: Bobbie Lind <bobbie.j.lind@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>