TARGET_CPUS is the default irq routing poicy.  It specifies which cpus the
kernel should aim an irq at.  In physflat delivery mode we can route an irq to
a single cpu.  But that doesn't mean our default policy should only be a
single cpu is allowed.

By allowing the irq routing code to select from multiple cpus this enables
systems with more irqs then we can service on a single processor to actually
work.

I just audited and tested the code and irqbalance doesn't care, and the
io_apic.c doesn't care if we have extra cpus in the mask.  Everything will use
or assume we are using the lowest numbered cpu in the mask if we can't use
them all.

So this should result in no behavior changes except on systems that need it.

Thanks for YH Lu for spotting this problem in his testing.

Cc: Yinghai Lu <yinghai.lu@amd.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Andi Kleen <ak@suse.de>