FindIp reads pcap packets until one is found that matches the given MAC and contains an IP address.

IOSPacketHeader default is -1.

IOSPacketHeader :) ref: https://github.com/gofmt/iOSSniffer/blob/master/pkg/sniffer/sniffer.go#L44.

ref: https://www.wireshark.org/~martinm/mac_pcap_sample_code.c.