Merge branch 'rcu.2025.09.23a' into HEAD

RCU miscellaneous updates:
* Document that rcu_barrier() hurries lazy callbacks
* Remove local_irq_save/restore() in rcu_preempt_deferred_qs_handler()
* Move list_for_each_rcu() to where it belongs
* Replace use of system_wq with system_percpu_wq
* WQ_PERCPU added to alloc_workqueue users
* WQ_UNBOUND added to sync_wq workqueue