Add sysfs file to map PCI busses to cpus

Export the information from pcibus_to_cpumask() to sysfs. This
is useful for some user space programs who want to optimize their IO
using O_DIRECT.

There was some indecision on whether it's more useful to report
cpus or nodes here. In the end cpus was chosen because that is
what the existing macros report.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>