Revert "cgroup/cpuset: Convert cpuset_mutex to percpu_rwsem"

This reverts commit 1243dc51.

percpu_rwsem is an rcu based lock.  Under loaded conditions
this lock will require that each cpu perform a switch, causing
exessive delays with long-running tasks on those CPUs.

In the case of hotplug/pause, this can slow down the ability
to activate/deactivate a CPU.

Revert the change from cpuset_mutex to percpu_rwsem, to
eliminate that overhead, and revert to a global lock.

Bug: 161210528
Change-Id: Id00dcaa6d601b561d1321d3e944b6c52e9663f1a
Signed-off-by: Stephen Dickey <dickey@codeaurora.org>