bpf: Allow skb_ancestor_cgroup_id helper in cgroup skb

bpf: Allow skb_ancestor_cgroup_id helper in cgroup skb

cgroup skb programs already can use bpf_skb_cgroup_id. Allow
bpf_skb_ancestor_cgroup_id as well so that container policies can be
implemented for a container that can have sub-cgroups dynamically
created, but policies should still be implemented based on cgroup id of
container itself not on an id of a sub-cgroup.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/8874194d6041eba190356453ea9f6071edf5f658.1589486450.git.rdna@fb.com