sched_ext: Fix possible deadlock in the deferred_irq_workfn()

In the Linux kernel, the following vulnerability has been resolved:

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);

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.