selftests/net: Add bpf skb forwarding program

selftests/net: Add bpf skb forwarding program

Add nk_forward.bpf.c, a BPF program that forwards skbs matching some IPv6
prefix received on eth0 ifindex to a specified netkit ifindex. This will
be needed by netkit container tests.

Signed-off-by: David Wei <dw@davidwei.uk>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260305181803.2912736-2-dw@davidwei.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>