workqueue: Disable per-cpu CPU hog detection when wq_cpu_intensive_thresh_us is 0

If workqueue.cpu_intensive_thresh_us is set to 0, the detection mechanism
for CPU-hogging per-cpu work item will keep triggering spuriously:

  workqueue: process_srcu hogged CPU for >0us 4 times, consider switching to WQ_UNBOUND
  workqueue: gc_worker hogged CPU for >0us 4 times, consider switching to WQ_UNBOUND
  workqueue: gc_worker hogged CPU for >0us 8 times, consider switching to WQ_UNBOUND
  workqueue: wait_rcu_exp_gp hogged CPU for >0us 4 times, consider switching to WQ_UNBOUND
  workqueue: kfree_rcu_monitor hogged CPU for >0us 4 times, consider switching to WQ_UNBOUND
  workqueue: kfree_rcu_monitor hogged CPU for >0us 8 times, consider switching to WQ_UNBOUND
  workqueue: reg_todo hogged CPU for >0us 4 times, consider switching to WQ_UNBOUND

This commit therefore disables the CPU-hog detection mechanism when
workqueue.cpu_intensive_thresh_us is set to 0.

tj: Patch description updated and the condition check on
    cpu_intensive_thresh_us separated into a separate if statement for
    readability.

Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>