Don't treat IPv4-mapped link-local IP addresses as IPv6 link-local
addresses, an IPv4 broadcast address as a global unicast IP address.

Fixes #11585.

Change-Id: I6a7a0c0601f18638f5c624ab63e12ee40f77b182
Reviewed-on: https://go-review.googlesource.com/11883Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>