MDEV-27754 : Assertion with innodb_flush_method=O_DSYNC

If innodb_flush_method=O_DSYNC, log_sys.flushed_to_disk_lsn  is changed
without 'flush_lock' protection inside log_write().

This leads to a race condition, if there are 2 threads running in parallel,
doing log_write_up_to() with different values for 'flush_to_disk'

In this case, log_write() and log_write_flush_to_disk_low() can execute at
the same time, and both would change flushed_lsn.

The fix is to remove special treatment of durable writes from log_write().
There is no apparent reason for this special treatment, log_write_flush_to_disk_low()
is already optimized for durable writes.

Nor there is an apparent reason to call log_flush_notify() more often in
for O_DSYNC.

storage/innobase/log/log0log.cc

@@ -639,8 +639,7 @@ log_write_buf(
 	}
 }
 
-/** Flush the recently written changes to the log file.
-and invoke mysql_mutex_lock(&log_sys.mutex). */
+/** Flush the recently written changes to the log file.*/
 static void log_write_flush_to_disk_low(lsn_t lsn)
 {
   if (!log_sys.log.writes_are_durable())
@@ -780,10 +779,6 @@ static void log_write(bool rotate_key)
 		start_offset - area_start);
 	srv_stats.log_padded.add(pad_size);
 	log_sys.write_lsn = write_lsn;
-	if (log_sys.log.writes_are_durable()) {
-		log_sys.set_flushed_lsn(write_lsn);
-		log_flush_notify(write_lsn);
-	}
 	return;
 }