Now that sockmap/sockhash can hold listening sockets, when setting up the
psock we will (i) grab references to verdict/parser progs, and (2) override
socket upcalls sk_data_ready and sk_write_space.

However, since we cannot redirect to listening sockets so we don't need to
link the socket to the BPF progs. And more importantly we don't want the
listening socket to have overridden upcalls because they would get
inherited by child sockets cloned from it.

Introduce a separate initialization path for listening sockets that does
not change the upcalls and ignores the BPF progs.
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200218171023.844439-6-jakub@cloudflare.com