net/flow_dissector: pass flow_keys->n_proto to BPF programs

This is a preparation for the next commit that would prohibit access to
the most fields of __sk_buff from the BPF programs.

Instead of requiring BPF flow dissector programs to look into skb,
pass all input data in the flow_keys.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>