selftests/bpf: add sockopt clone/inheritance test

Add a test that calls setsockopt on the listener socket which triggers
BPF program. This BPF program writes to the sk storage and sets
clone flag. Make sure that sk storage is cloned for a newly
accepted connection.

We have two cloned maps in the tests to make sure we hit both cases
in bpf_sk_storage_clone: first element (sk_storage_alloc) and
non-first element(s) (selem_link_map).

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