Revert "cgroup: Move rcu_head up near the top of cgroup_root"

This reverts commit 9405d778 which is
commit a7fb0423 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I6c3edaa3061ad36754f236e350909159def22f11
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>