bpf: remove cg_skb_func_proto and use sk_filter_func_proto directly

Since cg_skb_func_proto() doesn't do anything else than just calling
into sk_filter_func_proto(), remove it and set sk_filter_func_proto()
directly for .get_func_proto callback.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>