MDEV-25313: Assertion pending==log_requests.start... failed

log_flush_notify(): Restore the reload of log_requests.start that was
accidentally removed in commit 8c2e3259.

Thanks to Elena Stepanova for a test case (repeatedly running FLUSH LOGS
concurrently with InnoDB write transactions).

storage/innobase/handler/ha_innodb.cc

@@ -4506,6 +4506,7 @@ void log_flush_notify(lsn_t flush_lsn)
   if (auto pending= log_requests.start.load(std::memory_order_acquire))
   {
     mysql_mutex_lock(&log_requests.mutex);
+    pending= log_requests.start.load(std::memory_order_relaxed);
     log_flush_notify_and_unlock(pending, flush_lsn);
   }
 }