sched_ext: Fix possible deadlock in the deferred_irq_workfn()

sched_ext: Fix possible deadlock in the deferred_irq_workfn()

For PREEMPT_RT=y kernels, the deferred_irq_workfn() is executed in
the per-cpu irq_work/* task context and not disable-irq, if the rq
returned by container_of() is current CPU's rq, the following scenarios
may occur:

lock(&rq->__lock);
<Interrupt>
  lock(&rq->__lock);

This commit use IRQ_WORK_INIT_HARD() to replace init_irq_work() to
initialize rq->scx.deferred_irq_work, make the deferred_irq_workfn()
is always invoked in hard-irq context.

Signed-off-by: Zqiang <qiang.zhang@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>