show nohz_full cpus in sysfs

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 <[email protected]>
Acked-by: Mike Galbraith <[email protected]>
Acked-by: Chris Metcalf <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>