irq: clear kstat_irqs

Impact: get correct kstat_irqs [/proc/interrupts] for msi/msi-x etc

need to call clear_kstat_irqs(), so when we reuse that irq_desc,
we get correct kstat in /proc/interrupts.

This makes /proc/interrupts not have <NULL> entries.

Don't need to worry about arch that doesn't support genirq, because they
will not call dynamic_irq_cleanup().

v2: simplify and make clear_kstat_irqs more robust

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>