cgroup: add cgroup_subsys->free() method and use it to fix pids controller

pids controller is completely broken in that it uncharges when a task
exits allowing zombies to escape resource control.  With the recent
updates, cgroup core now maintains cgroup association till task free
and pids controller can be fixed by uncharging on free instead of
exit.

This patch adds cgroup_subsys->free() method and update pids
controller to use it instead of ->exit() for uncharging.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>