selftests/bpf: convert socket_cookie test to sk storage

This lets us test that both BPF_PROG_TYPE_CGROUP_SOCK_ADDR and
BPF_PROG_TYPE_SOCK_OPS can access underlying bpf_sock.

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