irqwork: Move irq safe work to irq context

On architectures where arch_irq_work_has_interrupt() returns false, we
end up running the irq safe work from the softirq context. That
results in a potential deadlock in the scheduler irq work which
expects that function to be called with interrupts disabled.

Split the irq_work_tick() function into a hard and soft variant. Call
the hard variant from the tick interrupt and add the soft variant to
the timer softirq.

Reported-and-tested-by: Yanjiang Jin <yanjiang.jin@windriver.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable-rt@vger.kernel.org