workqueue: Add system_dfl_long_wq for long unbound works

Currently there are users of queue_delayed_work() who specify
system_long_wq, the per-cpu workqueue. This workqueue should
be used for long per-cpu works, but queue_delayed_work()
queue the work using:

queue_delayed_work_on(WORK_CPU_UNBOUND, ...);

This would end up calling __queue_delayed_work() that does:

if (housekeeping_enabled(HK_TYPE_TIMER)) {
// [....]
} else {
if (likely(cpu == WORK_CPU_UNBOUND))
add_timer_global(timer);
else
add_timer_on(timer, cpu);
}

So when cpu == WORK_CPU_UNBOUND the timer is global and is
not using a specific CPU. Later, when __queue_work() is called:

if (req_cpu == WORK_CPU_UNBOUND) {
if (wq->flags & WQ_UNBOUND)
cpu = wq_select_unbound_cpu(raw_smp_processor_id());
else
cpu = raw_smp_processor_id();
}

Because the wq is not unbound, it takes the CPU where the timer
fired and enqueue the work on that CPU.
The consequence of all of this is that the work can run anywhere,
depending on where the timer fired.

Introduce system_dfl_long_wq in order to change, in a future step,
users that are still calling:

queue_delayed_work(system_long_wq, ...);

with the new system_dfl_long_wq instead, so that the work may
benefit from scheduler task placement.

Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>