genirq: Implement handle_irq_event()

Core code replacement for the ugly camel case. It contains all the
code which is shared in all handlers.

     clear status flags
     set INPROGRESS flag
     unlock
     call action chain
     note_interrupt
     lock
     clr INPROGRESS flag

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>