bpf: sockmap sample, report bytes/sec

Report bytes/sec sent as well as total bytes. Useful to get rough
idea how different configurations and usage patterns perform with
sockmap.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>