UPSTREAM: cgroup: Simplify cgroup_ancestor

Simplify cgroup_ancestor function. This is follow-up for
commit 77236281 ("bpf: Introduce bpf_skb_ancestor_cgroup_id helper")

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Bug: 154548692
Signed-off-by: Marco Ballesio <balejs@google.com>
(cherry picked from commit 808c43b7c7f70360ed7b9e43e2cf980f388e71fa)
Change-Id: I246d9ab057805abd0fa54ed1c23a15c0c2caf552