net: adjust the test for IPv4 loopback address block

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>

src/net/interface_test.go

@@ -202,7 +202,7 @@ func validateInterfaceUnicastAddrs(ifat []Addr) (*routeStats, error) {
 				if 0 >= prefixLen || prefixLen > 8*IPv4len || maxPrefixLen != 8*IPv4len {
 					return nil, fmt.Errorf("unexpected prefix length: %d/%d for %#v", prefixLen, maxPrefixLen, ifa)
 				}
-				if ifa.IP.IsLoopback() && (prefixLen != 8 && prefixLen != 8*IPv4len) { // see RFC 1122
+				if ifa.IP.IsLoopback() && prefixLen < 8 { // see RFC 1122
 					return nil, fmt.Errorf("unexpected prefix length: %d/%d for %#v", prefixLen, maxPrefixLen, ifa)
 				}
 				stats.ipv4++