bpf: net: Set sk_bpf_storage back to NULL for cloned sk

The cloned sk should not carry its parent-listener's sk_bpf_storage.
This patch fixes it by setting it back to NULL.

Fixes: 6ac99e8f ("bpf: Introduce bpf sk local storage")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>