icmp: simplify Message.Marshal
icmp: simplify Message.Marshal

Change-Id: I1c78c929dd824534ccc2123a63973af89b8a0081
Reviewed-on: https://go-review.googlesource.com/c/155857
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/net