dns/dnsmessage: gofmt -w -s

Change-Id: Iba4cf005f521a1ab7ff721f023222c364056af20
Reviewed-on: https://go-review.googlesource.com/122416
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>