cgroup: Fixes for v7.1-rc2

- During v6.19, cgroup task unlink was moved from do_exit() to after the
  final task switch to satisfy a controller invariant. That left the kernel
  seeing tasks past exit_signals() longer than userspace expected, and
  several v7.0 follow-ups tried to bridge the gap by making rmdir wait for
  the kernel side. None held up. The latest is an A-A deadlock when rmdir
  is invoked by the reaper of zombies whose pidns teardown the rmdir itself
  is waiting on, which points at the synchronizing approach being
  fundamentally wrong:

  - Take a different approach: drop the wait, leave rmdir's user-visible
    side returning as soon as cgroup.procs is empty, and defer the css
    percpu_ref kill that drives ->css_offline() until the cgroup is fully
    depopulated.

  - Tagged for stable. Somewhat invasive but contained. The hope is that
    fixing forward sticks. If not, the fallback is to revert the entire
    chain and rework on the development branch.

  - Doesn't plug a pre-existing analogous race in
    cgroup_apply_control_disable() (controller disable via subtree_control).
    Not a regression. The development branch will do the more invasive
    restructuring needed for that.

- Documentation update for cgroup-v1 charge-commit section that still
  referenced functions removed when the memcg hugetlb try-commit-cancel
  protocol was retired.

docs: cgroup-v1: Update charge-commit section

Commit 1d8f136a421f ("memcg/hugetlb: remove memcg hugetlb
try-commit-cancel protocol") removed mem_cgroup_commit_charge() and
mem_cgroup_cancel_charge(), but the docs still refer to those functions.
There is no longer any charge cancellation.

Update the docs to match the code.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>