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>