net: make use of IPv4len, IPv6len

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4926046
diff --git a/src/pkg/net/udpsock_posix.go b/src/pkg/net/udpsock_posix.go
index dfda997..1dc79f7 100644
--- a/src/pkg/net/udpsock_posix.go
+++ b/src/pkg/net/udpsock_posix.go
@@ -23,7 +23,7 @@
 }
 
 func (a *UDPAddr) family() int {
-	if a == nil || len(a.IP) <= 4 {
+	if a == nil || len(a.IP) <= IPv4len {
 		return syscall.AF_INET
 	}
 	if a.IP.To4() != nil {