bpf: add get_netns_cookie helper to tc 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/20241007095958.97442-1-mahe.tardy@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>