MDEV-15914: buf_pool_is_obsolete(): Add UNIV_UNLIKELY

storage/innobase/include/buf0buf.ic

@@ -2,7 +2,7 @@
 
 Copyright (c) 1995, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
-Copyright (c) 2014, 2017, MariaDB Corporation.
+Copyright (c) 2014, 2018, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -1424,8 +1424,8 @@ bool
 buf_pool_is_obsolete(
 	ulint	withdraw_clock)
 {
-	return(buf_pool_withdrawing
-	       || buf_withdraw_clock != withdraw_clock);
+	return(UNIV_UNLIKELY(buf_pool_withdrawing
+			     || buf_withdraw_clock != withdraw_clock));
 }
 
 /** Calculate aligned buffer pool size based on srv_buf_pool_chunk_unit,