ipv4: move unexposed error values into helper.go

Change-Id: I86f7d9e18f9d51f0a92ce32690a9c4aab1c79ab4
Reviewed-on: https://go-review.googlesource.com/16320
Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv4/header.go

@@ -5,7 +5,6 @@
 package ipv4
 
 import (
-	"errors"
 	"fmt"
 	"net"
 	"runtime"
@@ -13,14 +12,6 @@ import (
 	"unsafe"
 )
 
-var (
-	errMissingAddress  = errors.New("missing address")
-	errMissingHeader   = errors.New("missing header")
-	errHeaderTooShort  = errors.New("header too short")
-	errBufferTooShort  = errors.New("buffer too short")
-	errInvalidConnType = errors.New("invalid conn type")
-)
-
 const (
 	Version      = 4  // protocol version
 	HeaderLen    = 20 // header length without extension headers

ipv4/helper.go

@@ -10,6 +10,11 @@ import (
 )
 
 var (
+	errMissingAddress           = errors.New("missing address")
+	errMissingHeader            = errors.New("missing header")
+	errHeaderTooShort           = errors.New("header too short")
+	errBufferTooShort           = errors.New("buffer too short")
+	errInvalidConnType          = errors.New("invalid conn type")
 	errOpNoSupport              = errors.New("operation not supported")
 	errNoSuchInterface          = errors.New("no such interface")
 	errNoSuchMulticastInterface = errors.New("no such multicast interface")