s390: Replace use of system_wq with system_dfl_wq

Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistentcy cannot be addressed without refactoring the API.

system_wq is a per-CPU worqueue (replaced by system_percpu_wq), but the
current code does not benefit from it. Because of that, system_wq has been
replaced by system_dfl_wq, the new unbound workqueue.

The old wq will be kept for a few release cylces.

Suggested-by: Tejun Heo <[e-mail]>
Signed-off-by: Marco Crivellari <[e-mail]>
Acked-by: Heiko Carstens <[e-mail]>
Reviewed-by: Mete Durlu <[e-mail]>
Signed-off-by: Alexander Gordeev <[e-mail]>

arch/s390/kernel/hiperdispatch.c

···
 		return 0;
 	if (hd_online_cores <= hd_entitled_cores)
 		return 0;
-	mod_delayed_work(system_wq, &hd_capacity_work, HD_DELAY_INTERVAL * hd_delay_factor);
+	mod_delayed_work(system_dfl_wq, &hd_capacity_work, HD_DELAY_INTERVAL * hd_delay_factor);
 	hd_update_capacities();
 	return 1;
 }