selftests/bpf: Verify socket storage in cgroup/sock_{create, release}

Augment udp_limit test to set and verify socket storage value.
That should be enough to exercise the changes from the previous
patch.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200729003104.1280813-2-sdf@google.com