driver core: replace use of system_unbound_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.

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

commit 128ea9f ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea ("workqueue: Add new WQ_PERCPU flag")

Switch to using system_dfl_wq because system_unbound_wq is going away as part of
a workqueue restructuring.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Link: https://patch.msgid.link/20251114141618.172154-2-marco.crivellari@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: DispatchCode

drivers/base/dd.c

@@ -193,7 +193,7 @@ void driver_deferred_probe_trigger(void)
 	 * Kick the re-probe thread.  It may already be scheduled, but it is
 	 * safe to kick it again.
 	 */
-	queue_work(system_unbound_wq, &deferred_probe_work);
+	queue_work(system_dfl_wq, &deferred_probe_work);
 }
 
 /**