BACKPORT: 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>
Change-Id: I9e96704713f34fbc68e92b9f91c01b593708220f
Bug: 154548692
Signed-off-by: Marco Ballesio <balejs@google.com>
(cherry picked from commit 808c43b7c7f70360ed7b9e43e2cf980f388e71fa)
This cherry pick differs from the original in that cgroup_ancestor is added
in place of being just modified. The patch originally introducing the
function was 77236281 (bpf: Introduce bpf_skb_ancestor_cgroup_id
helper) which also relied on bpf dependencies not present in
android-4.14. cgroup_ancestor is independent from the bpf_skb code and
can hence be taken alone