selftests/bpf: Fix size of map_fd in test_sockmap

The array size of map_fd[] is 9, not 8. This patch changes it as a more
general form: ARRAY_SIZE(map_fd).

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/0972529ee01ebf8a8fd2b310bdec90831c94be77.1716446893.git.tanggeliang@kylinos.cn