Bug#20422680 BUF_POOL_WATCH_SET WOULD CRASH TRYING

			TO USE A SECOND WATCH PAGE PER INSTANCE

Description:
	BUF_POOL_WATCH_SIZE is also initialized to number of purge threads.
so BUF_POOL_WATCH_SIZE will never be lesser than number of purge threads.
From the code, there is no scope for purge thread to skip buf_pool_watch_unset.
So there can be at most one buffer pool watch active per purge thread.
In other words, there is no chance for purge thread instance to hold a watch
when setting another watch.

Solution:
	Adding code comments to clarify the issue.
Reviewed-by: Marko Mäkelä <marko.makela@oracle.com>
Approved via Bug page.

storage/innobase/buf/buf0buf.c

 /*****************************************************************************
 
-Copyright (c) 1995, 2014, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 1995, 2015, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
 
 Portions of this file contain modifications contributed and copyrighted by
@@ -1528,6 +1528,9 @@ buf_pool_watch_set(
 		return(NULL);
 	}
 
+	/* The maximum number of purge threads should never exceed
+	BUF_POOL_WATCH_SIZE. So there is no way for purge thread
+	instance to hold a watch when setting another watch. */
 	for (i = 0; i < BUF_POOL_WATCH_SIZE; i++) {
 		bpage = &buf_pool->watch[i];