bpf: add get_netns_cookie helper to cgroup_skb programs

This is needed in the context of Cilium and Tetragon to retrieve netns
cookie from hostns when traffic leaves Pod, so that we can correlate
skb->sk's netns cookie.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
Link: https://lore.kernel.org/r/20250225125031.258740-1-mahe.tardy@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>