go.net/ipv4: diable the use of ip_mreqn on freebsd

This CL disables the use of ip_mreqn with IP_{ADD,DROP}_MEMBERSHIP on
FreeBSD due to some test flakiness on some kernel versions that support
IGMPv3. It will be fixed once we support IGMPv3 related features such
as source-specific multicasting and source filtering.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/94670043

ipv4/sockopt_mreq.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly netbsd openbsd
+// +build darwin dragonfly freebsd netbsd openbsd
 
 package ipv4
 

ipv4/sockopt_mreqn.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build freebsd linux
+// +build linux
 
 package ipv4
 

ipv4/sys_mreq.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly netbsd openbsd windows
+// +build darwin dragonfly freebsd netbsd openbsd windows
 
 package ipv4