If a SYN cookie is not issued by tcp_v#_gen_syncookie, then the return
value will be exactly 0, rather than <= 0. Let's change the check to
reflect that, especially since mss is an unsigned value and cannot be
negative.
Fixes: 70d66244317e ("bpf: add bpf_tcp_gen_syncookie helper") Reported-by: Stanislav Fomichev <sdf@google.com> Signed-off-by: Petar Penkov <ppenkov@google.com> Acked-by: Song Liu <songliubraving@fb.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>