bpf: samples/sockmap detach sock ops program

samples/sockops program keeps the sock_ops program attached to cgroup.
Fixed this by detaching program before exit.

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>