samples/bpf: detach prog from cgroup

test_cgrp2_sock.sh and test_cgrp2_sock2.sh tests keep the program
attached to cgroup even after completion.
Using detach functionality of test_cgrp2_sock in both scripts.

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>