bpf: Add test for SO_REUSEPORT_DETACH_BPF

This patch adds a test for the new sockopt SO_REUSEPORT_DETACH_BPF.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>