icmp: use socket.NativeEndian

Change-Id: I991e893a0f0b0aea680ef18e09008b30e8a972c6
Reviewed-on: https://go-review.googlesource.com/46231
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
3 files changed