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: I7469fe330d49867bbdbdfae128834140244925ec
Reviewed-on: https://go-review.googlesource.com/30177
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>