x/net/ipv6: fix a nit found by vet

LGTM=dsymonds
R=iant, dsymonds
CC=golang-codereviews
https://golang.org/cl/176920043

ipv6/readwrite_test.go

@@ -92,7 +92,7 @@ func benchmarkReadWriteIPv6UDP(b *testing.B, p *ipv6.PacketConn, wb, rb []byte,
 	if n, err := p.WriteTo(wb, &cm, dst); err != nil {
 		b.Fatal(err)
 	} else if n != len(wb) {
-		b.Fatal("got %v; want %v", n, len(wb))
+		b.Fatalf("got %v; want %v", n, len(wb))
 	}
 	if _, _, _, err := p.ReadFrom(rb); err != nil {
 		b.Fatal(err)