cpuset: Make cpusets restore on hotplug

This deliberately changes the behavior of the per-cpuset
cpus file to not be effected by hotplug. When a cpu is offlined,
it will be removed from the cpuset/cpus file. When a cpu is onlined,
if the cpuset originally requested that that cpu was part of the cpuset, that
cpu will be restored to the cpuset. The cpus files still
have to be hierachical, but the ranges no longer have to be out of
the currently online cpus, just the physically present cpus.

Change-Id: Ie5e524698518fdc0ef4bc9851bd2fa798ac491d2
(cherry picked from commit 256ad63778fc0e2da38a137e30f3c6d2bd076b72)