Fix leaking freezer cgroups.

Continuation of 923c9b37.

There is a delay in the cgroup system when moving a PID from one cgroup
to another. It is usually short, but if we don't wait for the PID to
move before removing cgroup directories the PID previously belonged to,
we could leak cgroups. This was previously fixed in the cpuset and
devices subsystems. This uses the same logic to fix the freezer
subsystem.

Bug 5082.