workqueue: Simplify goto statement

Use a simple if-statement to replace the cumbersome goto-statement in
workqueue_set_unbound_cpumask().

Cc: Waiman Long <longman@redhat.com>
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Tejun Heo <tj@kernel.org>