cgroup: Fixes for v6.4-rc7

It's late but here are two bug fixes. Both fix problems which can be severe
but are very confined in scope. The risk to most use cases should be
minimal.

* Fix for an old bug which triggers if a cgroup subsystem is remounted to a
  different hierarchy while someone is reading its cgroup.procs/tasks file.
  The risk is pretty low given how seldom cgroup subsystems are moved across
  hierarchies.

* We moved cpus_read_lock() outside of cgroup internal locks a while ago but
  forgot to update the legacy_freezer leading to lockdep triggers. Fixed.
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