It looks like the latest DragonFly BSD kernels, at least 4.4 and above,
have finished working on handling of shared IP control blocks. Let's
re-enbale test cases referring to IP control blocks and see what
happens.

Updates #13146.

Change-Id: Icbe2250e788f6a445a648541272c99b598c3013d
Reviewed-on: https://go-review.googlesource.com/19406Reviewed-by: Ian Lance Taylor <iant@golang.org>