show nohz_full cpus in sysfs

Currently there is no way to query which CPUs are in nohz_full
mode from userspace.

Export the CPU list running in nohz_full mode in sysfs,
specifically in the file /sys/devices/system/cpu/nohz_full

This can be used by system management tools like libvirt,
openstack, and others to ensure proper task placement.

Signed-off-by: Rik van Riel <riel@redhat.com>
Acked-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Acked-by: Chris Metcalf <cmetcalf@ezchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>