selftests/bpf: use section names understood by libbpf in test_sockmap

selftests/bpf: use section names understood by libbpf in test_sockmap

libbpf can deduce program type and attach type from the ELF section name.
We don't need to pass it out-of-band if we switch to libbpf convention [1].

[1] https://docs.kernel.org/bpf/libbpf/program_types.html

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240522080936.2475833-1-jakub@cloudflare.com