CIDRMask just returns a mask which corresponds to an address
prefix in CIDR nonation. A subnet for an IPv6 mask sounds a bit
confusing.

Change-Id: Ic7859ce992bc2de4043d3b25caf9a1051d118b0e
Reviewed-on: https://go-review.googlesource.com/33262Reviewed-by: Ian Lance Taylor <iant@golang.org>