We live in the era of virtualization and isolation.
There is no reason to hesitate to use IPv4 loopback address block for
umbrella-type customer accommodating services.

Fixes #23903

Change-Id: I990dd98e2651a993dac1b105c0bc771f8631cb93
Reviewed-on: https://go-review.googlesource.com/95336Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>