Fix busy-loop in withQueueLock() on non-EEXIST mkdir failure

If mkdir(queue.lock) failed for a reason other than "already exists"
(e.g. a permissions or disk problem on the data directory), is_dir()
was false, so the loop `continue`d before ever reaching the tries>=40
retry-limit check or the usleep(), spinning at full CPU forever
instead of failing with a clear error.

Move the staleness check inside an is_dir() guard instead of using
continue to skip past the retry-limit logic, so the bounded-retry
behavior applies regardless of why mkdir failed.

exec/lib/BackupQueueService.php

@@ -1295,13 +1295,12 @@ final class BackupQueueService
         $tries = 0;
         while (!@mkdir($lockDir, 0700)) {
             $tries++;
-            if (!is_dir($lockDir)) {
-                continue;
-            }
 
-            $mtime = (int)@filemtime($lockDir);
-            if ($mtime > 0 && (time() - $mtime) > 120) {
-                @rmdir($lockDir);
+            if (is_dir($lockDir)) {
+                $mtime = (int)@filemtime($lockDir);
+                if ($mtime > 0 && (time() - $mtime) > 120) {
+                    @rmdir($lockDir);
+                }
             }
 
             if ($tries >= 40) {