cgroup: add cgroup_v1v2_get_from_[fd/file]()

Add cgroup_v1v2_get_from_fd() and cgroup_v1v2_get_from_file() that
support both cgroup1 and cgroup2.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>