x86/irq: Reorganize the search in assign_irq_vector

Split out the code which advances the target cpu for the search so we can
reuse it for the next patch which adds an early validation check for the
vectormask which we get from the apic.

Add comments while at it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Borislav Petkov <bp@alien8.de>
Tested-by: Joe Lawrence <joe.lawrence@stratus.com>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Jeremiah Mahler <jmmahler@gmail.com>
Cc: andy.shevchenko@gmail.com
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org #4.3+
Link: http://lkml.kernel.org/r/20151231160106.484562040@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>