Revert "cgroup: Make operations on the cgroup root_list RCU safe"

This reverts commit dd9542ae which is
commit d23b5c57 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: Ia7245694cbdf47e4bf9410f21ff4676f2f5ac528
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>