x86/irq: Factor out handler invocation from common_interrupt()

Prepare for calling external interrupt handlers directly from the posted
MSI demultiplexing loop. Extract the common code from common_interrupt() to
avoid code duplication.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240423174114.526704-8-jacob.jun.pan@linux.intel.com