ipv6: deduplicate socket option code

Change-Id: I836cdf68901dc77bc4c086cabd9d43fd033fbf1d
Reviewed-on: https://go-review.googlesource.com/27736
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
7 files changed