selftests/bpf: Adapt sockmap update error handling

selftests/bpf: Adapt sockmap update error handling

Update sockmap_listen to accommodate the recent change in sockmap that
rejects unbound UDP sockets.

TCP: Reject unbound and bound (unless established or listening).
UDP: Accept only bound sockets.

While at it, migrate to ASSERT_* and enforce reverse xmas tree.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20260707-sockmap-lookup-udp-leak-v4-3-f878346f27ab@rbox.co
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>