refactor decodeICMPv4() method
refactor decodeICMPv4() method

* reduce code duplication by using a base layer method

vtolstov/gopacket,ross-williams/gopacket,google/gopacket,vtolstov/gopacket,LrsK/gopacket,adriantam/gopacket,levigross/gopacket,rouzier/gopacket,xuzhaokui/gopacket,LrsK/gopacket,rouzier/gopacket,google/gopacket,ross-williams/gopacket,xuzhaokui/gopacket,levigross/gopacket,skyportsystems/gopacket,LrsK/gopacket,vtolstov/gopacket,skyportsystems/gopacket,google/gopacket,adriantam/gopacket,rouzier/gopacket,skyportsystems/gopacket,levigross/gopacket,ross-williams/gopacket,xuzhaokui/gopacket,adriantam/gopacket