workqueue: fix opencoded cpumask_next_and_wrap() in wq_select_unbound_cpu()

The dedicated helper is more verbose and effective comparing to
cpumask_first() followed by cpumask_next().

Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>