libbpf: Fix probe for BPF_PROG_TYPE_CGROUP_SOCKOPT

This patch fixes the probe for BPF_PROG_TYPE_CGROUP_SOCKOPT,
so the probe reports accurate results when used by e.g.
bpftool.

Fixes: 4cdbfb59 ("libbpf: support sockopt hooks")
Signed-off-by: Robin Gögge <r.goegge@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20210728225825.2357586-1-r.goegge@gmail.com