genirq: honour default IRQ affinity setting during migration

Userspace can set the default IRQ affinity setting by writing into
/proc/irq/default_smp_affinity file. When an IRQ affinity is
broken during isolation/hotplug,override the affinity to online and
un-isolated CPUs from the default affinity CPUs. If no such CPU
is available, then only override with cpu_online_mask.

Change-Id: I7578728ed0d7c17c5890d9916cfd6451d1968568
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>