MDEV-33275 buf_flush_LRU(): mysql_mutex_assert_owner(&buf_pool.mutex) failed

In commit a55b951e (MDEV-26827)
an error was introduced in a rarely executed code path of the
buf_flush_page_cleaner() thread. As a result, the function
buf_flush_LRU() could be invoked while not holding buf_pool.mutex.

Reviewed by: Debarun Banerjee

storage/innobase/buf/buf0flu.cc

@@ -2547,6 +2547,8 @@ static void buf_flush_page_cleaner()
     else if (buf_flush_async_lsn <= oldest_lsn &&
              !buf_pool.need_LRU_eviction())
       goto check_oldest_and_set_idle;
+    else
+      mysql_mutex_lock(&buf_pool.mutex);
 
     n= srv_max_io_capacity;
     n= n >= n_flushed ? n - n_flushed : 0;