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

This reverts commit 047f0ee8 which is
commit a7fb0423c201ba12815877a0b5a68a6a1710b23a 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: I33ffbca6cd6d6d28cf37ca0256fcb75b1955a90c
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>