libbpf: Add bpf_link support for BPF_PROG_TYPE_SOCKMAP

Introduce a libbpf API function bpf_program__attach_sockmap()
which allow user to get a bpf_link for their corresponding programs.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240410043532.3737722-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>