selftests/bpf: Fail unbound UDP on sockmap update

selftests/bpf: Fail unbound UDP on sockmap update

sockmap now rejects unbound UDP sockets. Adjust test_maps. While at it,
check socket()'s return value.

This effectively reverts commit c39aa2159974 ("bpf, selftests: Fix
test_maps now that sockmap supports UDP").

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-v4-4-f878346f27ab@rbox.co
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>