The macOS kernel reliably crashes in a repeated TestRouteMessage.
Putting some extra padding into the request buffer avoids the crash.
This will do as workaround; the kernel bug will be reported to Apple separately.

Fixes golang/go#22456.

Change-Id: I789d3d57fbc511016d9f4a3fa7662d6c7642f137
Reviewed-on: https://go-review.googlesource.com/73690
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>