Fix bug in js for restoremailbox

For this action it appears that the mailboxid is the itemid, and that's
the only additional argument required for the underlying method
`VBO::restoreMailbox()`.

This surfaced as a javascript error due to the variable being undefined.

exchange.php

@@ -1204,6 +1204,7 @@ function restoreToOriginal(itemid, type) {
 									itemid = node[0].data.folderid;
 								} else {
 									var act = 'restoremailbox';
+                                    var mailboxid = itemid;
 								}
 
 								var json = '{ "restoretoOriginallocation": \
@@ -1706,4 +1707,4 @@ function loadMessages(folderid, offset) {
 }
 ?>
 </body>
-</html>
+</html>