If del_timer() returns true, the timer was pending. This means it was started
by work->func() (note that __run_timers() clears timer_pending() before calling
timer->function). This in turn means that delayed_work_timer_fn() has already
called __queue_work(dwork), otherwise work->func() has no chance to run.
When del_timer() returns true and delayed_work_timer_fn() doesn't run we are
safe, this doesn't differ from del_timer_sync().