firewall: show implicit IPv6 block rule if IPv6 disabled

If IPv6 is not enabled in global networking settings, show the implicitly
generated IPv6 block rule in floating firewall rules.

(cherry picked from commit 7407fe8b)

src/www/firewall_rules.php

@@ -273,6 +273,28 @@ $( document ).ready(function() {
                   </tr>
                 </thead>
                 <tbody>
+<?php
+                // Show floating block IPv6 rule if IPv6 is globally disabled in system settings
+                if (!isset($config['system']['ipv6allow']) &&
+                        ($selected_if == 'FloatingRules')):
+?>
+                  <tr valign="top">
+                    <td>&nbsp;</td>
+                    <td><span class="glyphicon glyphicon-remove text-danger"></span></td>
+                    <td>IPv6 *</td>
+                    <td>*</td>
+                    <td class="hidden-xs hidden-sm">*</td>
+                    <td class="hidden-xs hidden-sm">*</td>
+                    <td class="hidden-xs hidden-sm">*</td>
+                    <td class="hidden-xs hidden-sm">*</td>
+                    <td class="hidden-xs hidden-sm">&nbsp;</td>
+                    <td><?=gettext("Block all IPv6 traffic");?></td>
+                    <td>
+                      <a href="system_advanced_network.php" title="<?=gettext("edit rule");?>" class="btn btn-default btn-xs"><span class="glyphicon glyphicon-pencil"></span></a>
+                    </td>
+                  </tr>
+<?php
+                endif; ?>
 <?php
                 // Show the anti-lockout rule if it's enabled, and we are on LAN with an if count > 1, or WAN with an if count of 1.
                 if (!isset($config['system']['webgui']['noantilockout']) &&