MDEV-27499 Performance regression in log_checkpoint_margin()

In commit 4c3ad244 (MDEV-27416)
an unnecessarily strict wait condition was introduced in the
function buf_flush_wait(). Most callers actually only care that
the pages have been flushed, not that a checkpoint has completed.

Only in the buf_flush_sync() call for log resizing, we might care
about the log checkpoint. But, in fact,
srv_prepare_to_delete_redo_log_file() is explicitly disabling
checkpoints. So, we can simply remove the unnecessary wait loop.

Thanks to Krunal Bauskar for reporting this performance regression
that we failed to repeat in our testing.

storage/innobase/buf/buf0flu.cc

 /*****************************************************************************
 
 Copyright (c) 1995, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2021, MariaDB Corporation.
+Copyright (c) 2013, 2022, MariaDB Corporation.
 Copyright (c) 2013, 2014, Fusion-io
 
 This program is free software; you can redistribute it and/or modify it under
@@ -1850,11 +1850,6 @@ static void buf_flush_wait(lsn_t lsn)
     my_cond_wait(&buf_pool.done_flush_list,
                  &buf_pool.flush_list_mutex.m_mutex);
   }
-
-  /* Wait for the checkpoint. */
-  while (buf_flush_sync_lsn)
-    my_cond_wait(&buf_pool.done_flush_list,
-                 &buf_pool.flush_list_mutex.m_mutex);
 }
 
 /** Wait until all persistent pages are flushed up to a limit.