workqueue: Simplify wq_calc_pod_cpumask() with wq_online_cpumask

Avoid relying on cpu_online_mask for wqattrs changes so that
cpus_read_lock() can be removed from apply_wqattrs_lock().

And with wq_online_cpumask, attrs->__pod_cpumask doesn't need to be
reused as a temporary storage to calculate if the pod have any online
CPUs @attrs wants since @cpu_going_down is not in the wq_online_cpumask.

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