workqueue: make sysfs of unbound kworker cpumask more clever

Some unfriendly component, such as dpdk, write the same mask to
unbound kworker cpumask again and again. Every time it write to
this interface some work is queue to cpu, even though the mask
is same with the original mask.

So, fix it by return success and do nothing if the cpumask is
equal with the old one.

Signed-off-by: Mengen Sun <mengensun@tencent.com>
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: Tejun Heo <tj@kernel.org>