Keep track of lost packets as reported by pcap_stats

Keep track of lost packets as reported by pcap_stats

On my machine, if I generate the following IPv6 flood:

ip -6 neigh add fe80::1234  dev eth0 lladdr 00:11:22:33:44:55
nc -6 -u fe80::1234%eth0 1234 < /dev/zero

The traffic reported by iftop is much less than the reality, as iftop
can't read packets off pcap fast enough. This patch only demonstrate
the problem, I haven't started to dig into the cause yet.