sched/cpuset: Bring back cpuset_mutex

commit 111cd11b upstream.

Turns out percpu_cpuset_rwsem - commit 1243dc51

 ("cgroup/cpuset:
Convert cpuset_mutex to percpu_rwsem") - wasn't such a brilliant idea,
as it has been reported to cause slowdowns in workloads that need to
change cpuset configuration frequently and it is also not implementing
priority inheritance (which causes troubles with realtime workloads).

Convert percpu_cpuset_rwsem back to regular cpuset_mutex. Also grab it
only for SCHED_DEADLINE tasks (other policies don't care about stable
cpusets anyway).

Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
[ Conflict in kernel/cgroup/cpuset.c due to pulling changes in functions
  or comments that don't exist on this branch. Remove a BUG_ON() for rwsem
  that doesn't exist on mainline. ]
Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>