check_disk: get_fs_usage hasn't been run if using groups

the reason why it still worked sometimes was fsu_blocks beeing uninitialized
which resulted in a true test sometimes.

plugins/check_disk.c

@@ -263,11 +263,11 @@ main (int argc, char **argv)
       } else if (fs_include_list && !np_find_name (fs_include_list, me->me_type)) {
         continue;
       }
-
-      stat_path(path);
-      get_fs_usage (me->me_mountdir, me->me_devname, &fsp);
     }
 
+    stat_path(path);
+    get_fs_usage (me->me_mountdir, me->me_devname, &fsp);
+
     if (fsp.fsu_blocks && strcmp ("none", me->me_mountdir)) {
       get_stats (path, &fsp);