selftests/bpf: Use bpf_link attachments in test_sockmap

selftests/bpf: Use bpf_link attachments in test_sockmap

Switch attachments to bpf_link using bpf_program__attach_sockmap() instead
of bpf_prog_attach().

This patch adds a new array progs[] to replace prog_fd[] array, set in
populate_progs() for each program in bpf object.

And another new array links[] to save the attached bpf_link. It is
initalized as NULL in populate_progs, set as the return valuses of
bpf_program__attach_sockmap(), and detached by bpf_link__detach().

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/32cf8376a810e2e9c719f8e4cfb97132ed2d1f9c.1716446893.git.tanggeliang@kylinos.cn