flow_dissector: Make 'bpf_flow_dissect' return the bpf program retcode

Let 'bpf_flow_dissect' callers know the BPF program's retcode and act
accordingly.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20220821113519.116765-2-shmulik.ladkani@gmail.com