smp: Use system_percpu_wq instead of system_wq

When a caller enqueues a work item using schedule_delayed_work() the used
wq is "system_wq" (per-cpu wq) while queue_delayed_work() uses
WORK_CPU_UNBOUND (used when no target CPU is specified). The same applies
to schedule_work() that is using system_wq and queue_work(), which again
makes use of WORK_CPU_UNBOUND.

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

Continue the effort to refactor workqueue APIs, which began with the
introduction of new workqueues and a new alloc_workqueue() flag in:

commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

and switch smp_call_on_cpu() to use system_percpu_wq because system_wq is
going away once the ongoing workqueue restructuring is done.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/20251110170332.319314-1-marco.crivellari@suse.com