Only apply RFC 6724's CommonPrefixLen rule for IPv4 source/destination
pairs that are members of the same IPv4 special purpose block.

Fixes #13283.

Change-Id: I2f7c26b408dd4675dfc5c1959e22d05b43bb8241
Reviewed-on: https://go-review.googlesource.com/16995Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>