ipv6: use socket.NativeEndian

Change-Id: Ia73c7bfff664f91c8a4c97656d0bd9e576145aa4
Reviewed-on: https://go-review.googlesource.com/46230
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed