bpf: add bpf_tcp_gen_syncookie helper

This helper function allows BPF programs to try to generate SYN
cookies, given a reference to a listener socket. The function works
from XDP and with an skb context since bpf_skc_lookup_tcp can lookup a
socket in both cases.

Signed-off-by: Petar Penkov <ppenkov@google.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>