selftests/net: test sk_filter support for SKF_NET_OFF on frags

selftests/net: test sk_filter support for SKF_NET_OFF on frags

Verify that a classic BPF linux socket filter correctly matches
packet contents. Including when accessing contents in an
skb_frag.

1. Open a SOCK_RAW socket with a classic BPF filter on UDP dport 8000.
2. Open a tap device with IFF_NAPI_FRAGS to inject skbs with frags.
3. Send a packet for which the UDP header is in frag[0].
4. Receive this packet to demonstrate that the socket accepted it.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20250408132833.195491-3-willemdebruijn.kernel@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>