flow_dissector: constify bpf_flow_dissector's data pointers

BPF Flow dissection programs are read-only and don't touch input
buffers.
Mark 'data' and 'data_end' in struct bpf_flow_dissector as const
in preparation for global input constifying.
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/flow_dissector.h

@@ -368,8 +368,8 @@ static inline void *skb_flow_dissector_target(struct flow_dissector *flow_dissec
 struct bpf_flow_dissector {
 	struct bpf_flow_keys	*flow_keys;
 	const struct sk_buff	*skb;
-	void			*data;
-	void			*data_end;
+	const void		*data;
+	const void		*data_end;
 };
 
 static inline void