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

This reverts commit 0475cf04.

Bring back upstream commit dd9542ae ("cgroup: Make operations on the
cgroup root_list RCU safe"). The ABI issues introduced by this patch are
addressed in a subsequent patch.

Bug: 379227997
Bug: 381949343
Change-Id: I136f602437262162b850d84d6750b2f0b5ccc856
Signed-off-by: Carlos Llamas <cmllamas@google.com>
(cherry picked from commit a3937e3e)