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

This reverts commit 4abf1841 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: I1561314f01cdd49442735838c4aae3e3da9232c6
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>