ipmr: Replace use of system_unbound_wq with system_dfl_wq

This patch continues the effort to refactor workqueue APIs, which has begun
with the changes introducing new workqueues and a new alloc_workqueue flag:

   commit 128ea9f6cc ("workqueue: Add system_percpu_wq and system_dfl_wq")
   commit 930c2ea566 ("workqueue: Add new WQ_PERCPU flag")

The point of the refactoring is to eventually alter the default behavior of
workqueues to become unbound by default so that their workload placement is
optimized by the scheduler.

Before that to happen, workqueue users must be converted to the better named
new workqueues with no intended behaviour changes:

   system_wq -> system_percpu_wq
   system_unbound_wq -> system_dfl_wq

This way the old obsolete workqueues (system_wq, system_unbound_wq) can be
removed in the future.

Cc: David Ahern <dsahern@kernel.org>
Cc: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260515135143.259669-2-marco.crivellari@suse.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/ipmr_base.c

@@ -39,7 +39,7 @@ static void __mr_free_table(struct work_struct *work)
 
 void mr_table_free(struct mr_table *mrt)
 {
-	queue_rcu_work(system_unbound_wq, &mrt->work);
+	queue_rcu_work(system_dfl_wq, &mrt->work);
 }
 
 struct mr_table *