cgroup: Fixes for v6.4-rc5

* Fix css_set reference leaks on fork failures.

* Fix CPU hotplug locking in cgroup_transfer_tasks() which is used by
  cgroup1 cpuset.

* Doc update.
-----BEGIN PGP SIGNATURE-----

iIQEABYIACwWIQTfIjM1kS57o3GsC/uxYfJx3gVYGQUCZIJ5bQ4cdGpAa2VybmVs
Lm9yZwAKCRCxYfJx3gVYGc/CAQCmE2cKGBWN45xbzIA5S7+zq8QCv85BYlnAgqpR
jgF8GQD/fFXdmKL0wTzjTf1YOvEi9UxJqhDvHSRtV53fzPedbg4=
=s+u8
-----END PGP SIGNATURE-----