MDEV-20805 ibuf_add_free_page() is not initializing FIL_PAGE_TYPE first

In the function recv_parse_or_apply_log_rec_body() there are debug checks
for validating the state of the page when redo log records are being
applied. Most notably, FIL_PAGE_TYPE should be set before anything else
is being written to the page.

ibuf_add_free_page(): Set FIL_PAGE_TYPE before performing any other changes.

storage/innobase/ibuf/ibuf0ibuf.c

 /*****************************************************************************
 
 Copyright (c) 1997, 2013, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -2222,14 +2223,14 @@ ibuf_add_free_page(void)
 	buf_block_dbg_add_level(block, SYNC_IBUF_TREE_NODE_NEW);
 	page = buf_block_get_frame(block);
 
+	mlog_write_ulint(page + FIL_PAGE_TYPE, FIL_PAGE_IBUF_FREE_LIST,
+			 MLOG_2BYTES, &mtr);
+
 	/* Add the page to the free list and update the ibuf size data */
 
 	flst_add_last(root + PAGE_HEADER + PAGE_BTR_IBUF_FREE_LIST,
 		      page + PAGE_HEADER + PAGE_BTR_IBUF_FREE_LIST_NODE, &mtr);
 
-	mlog_write_ulint(page + FIL_PAGE_TYPE, FIL_PAGE_IBUF_FREE_LIST,
-			 MLOG_2BYTES, &mtr);
-
 	ibuf->seg_size++;
 	ibuf->free_list_len++;
 

storage/xtradb/ibuf/ibuf0ibuf.c

 /*****************************************************************************
 
 Copyright (c) 1997, 2013, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -2266,14 +2267,14 @@ ibuf_add_free_page(void)
 	buf_block_dbg_add_level(block, SYNC_IBUF_TREE_NODE_NEW);
 	page = buf_block_get_frame(block);
 
+	mlog_write_ulint(page + FIL_PAGE_TYPE, FIL_PAGE_IBUF_FREE_LIST,
+			 MLOG_2BYTES, &mtr);
+
 	/* Add the page to the free list and update the ibuf size data */
 
 	flst_add_last(root + PAGE_HEADER + PAGE_BTR_IBUF_FREE_LIST,
 		      page + PAGE_HEADER + PAGE_BTR_IBUF_FREE_LIST_NODE, &mtr);
 
-	mlog_write_ulint(page + FIL_PAGE_TYPE, FIL_PAGE_IBUF_FREE_LIST,
-			 MLOG_2BYTES, &mtr);
-
 	ibuf->seg_size++;
 	ibuf->free_list_len++;