x86/PCI: default pcibus cpumask to all cpus if it lacks affinity

The early initialization of the pci bus to node mapping leaves all busses
with a node id of -1 if it lacks memory affinity.  Thus, cpumask_of_pcibus
must return all online cpus for such busses.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>