In case of selftest mode, temporary cgroup environment is created but
cgroup is not joined. It causes test failures. Fixed by joining the
cgroup

Fixes: 16962b24 ("bpf: sockmap, add selftests")
Acked-by: NJohn Fastabend <john.fastabend@gmail.com>
Signed-off-by: NPrashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>