ipv6: don't refer to cgo-generated union field

The size of sockaddr_storage is fixed for interoperability on each
platform. There is no need to depend on changable cgo-generated
offset values.

Change-Id: I54b775b2a8c86d969a6da0398a335075f0cbe3df
Reviewed-on: https://go-review.googlesource.com/30178
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed