i2c: testunit: Replace system_long_wq with system_dfl_long_wq

Currently the code enqueue work items using {queue|mod}_delayed_work(),
using system_long_wq. This workqueue should be used when long works are
expected, but it is a per-cpu workqueue.

This is important because queue_delayed_work() queue the work using:

queue_delayed_work_on(WORK_CPU_UNBOUND, ...);

Note that WORK_CPU_UNBOUND = NR_CPUS.

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.

Recently, a new unbound workqueue specific for long running work has
been added:

c116737e972e ("workqueue: Add system_dfl_long_wq for long unbound works")

So change system_long_wq with system_dfl_long_wq so that the work may
benefit from scheduler task placement.

Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
[wsa: remove FIXME as well]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>