wgengine/filter: fix copy/pasteo in new benchmark's v6 CIDR

I noticed the not-local-v6 numbers were nowhere near the v4 numbers
(they should be identical) and then saw this. It meant the
Addr().Next() wasn't picking an IP that was no longer local, as
assumed.

Updates #12486

Change-Id: I18dfb641f00c74c6252666bc41bd2248df15fadd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/filter/filter_test.go

@@ -1008,7 +1008,7 @@ func benchmarkFile(b *testing.B, file string, opt benchOpt) {
 	var localNets netipx.IPSetBuilder
 	pfx := []netip.Prefix{
 		netip.MustParsePrefix("100.96.14.120/32"),
-		netip.MustParsePrefix("fd7a:115c:a1e0:ab12:4843:cd96:6260:e78/32"),
+		netip.MustParsePrefix("fd7a:115c:a1e0:ab12:4843:cd96:6260:e78/128"),
 	}
 	for _, p := range pfx {
 		localNets.AddPrefix(p)