Fix go vet (#868)

firewall_test.go

@@ -755,7 +755,7 @@ func TestAddFirewallRulesFromConfig(t *testing.T) {
 	assert.Equal(t, addRuleCall{incoming: true, proto: firewall.ProtoAny, startPort: 1, endPort: 1, groups: nil, host: "a", ip: nil, localIp: nil}, mf.lastCall)
 
 	// Test adding rule with cidr
-	cidr := &net.IPNet{net.ParseIP("10.0.0.0").To4(), net.IPv4Mask(255, 0, 0, 0)}
+	cidr := &net.IPNet{IP: net.ParseIP("10.0.0.0").To4(), Mask: net.IPv4Mask(255, 0, 0, 0)}
 	conf = config.NewC(l)
 	mf = &mockFirewall{}
 	conf.Settings["firewall"] = map[interface{}]interface{}{"inbound": []interface{}{map[interface{}]interface{}{"port": "1", "proto": "any", "cidr": cidr.String()}}}