A framework to enqueueing and running callbacks in hardirq context.
文件：/include/linux/irq_work.h，/kernel/irq_work.c
/include/linux/irq_work.h
/kernel/irq_work.c
通过IPI发送中断，在中断上下文执行callback handler。
基于smp.c实现（IPI机制，ipi_msg_type: IPI_IRQ_WORK）