Add new helper bpf_sk_storage_clone which optionally clones sk storage
and call it from sk_clone_lock.

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