When I generalized __assign_irq_vector I failed to pay attention
to what happens when you access a per cpu data structure for
a cpu that is not online.   It is an undefined case making any
code that does it have undefined behavior as well.

The code still needs to be able to allocate a vector across cpus
that are not online to properly handle combinations like lowest
priority interrupt delivery and cpu_hotplug.  Not that we can do
that today but the infrastructure shouldn't prevent it.

So this patch updates the places where we touch per cpu data
to only touch online cpus, it makes cpu vector allocation
an atomic operation with respect to cpu hotplug, and it updates
the cpu start code to properly initialize vector_irq so we
don't have inconsistencies.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andi Kleen <ak@suse.de>