cgroup/cpuset: Miscellaneous cleanups & add helper functions

The partition root state (PRS) macro names do not currently match the
external names. Change them to match the external names and add helper
functions to read or change the state.

Shorten the cpuset argument of update_parent_subparts_cpumask() to cs
to match other cpuset functions.

Remove the new_prs argument from notify_partition_change() as the
cs->partition_root_state has already been set to new_prs before it
is called.

There is no functional change.
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>