refs codership/mysql-wsrep#233

- avoiding the race condition, by not grabbing thd->LOCK_wsrep_thd for
  accessing thd->wsrep_exec_mode. The caller is same thread and exec mode
  can only be changed by self.

sql/mdl.cc

@@ -1455,7 +1455,7 @@ MDL_wait::timed_wait(MDL_context_owner *owner, struct timespec *abs_timeout,
                    DBUG_ASSERT(!debug_sync_set_action((owner->get_thd()),
                                                       STRING_WITH_LEN(act)));
                  };);
-    if (wsrep_thd_is_BF(owner->get_thd(), true))
+    if (wsrep_thd_is_BF(owner->get_thd(), false))
     {
       wait_result= mysql_cond_wait(&m_COND_wait_status, &m_LOCK_wait_status);
     }