Fix description of WAL record XLOG_BTREE_META_CLEANUP

This record uses one metadata buffer and registers some data associated
to the buffer, but when parsing the record for its description a direct
access to the record data was done, but there is none.  This leads
usually to an incorrect description, but can also cause crashes like in
pg_waldump.  Instead, fix things so as the parsing uses the data
associated to the metadata block.

This is an oversight from 3d927961, so backpatch down to 11.

Author: Michael Paquier
Description: https://postgr.es/m/20190617013059.GA3153@paquier.xyz
Backpatch-through: 11

src/backend/access/rmgrdesc/nbtdesc.c

@@ -97,8 +97,10 @@ btree_desc(StringInfo buf, XLogReaderState *record)
 			}
 		case XLOG_BTREE_META_CLEANUP:
 			{
-				xl_btree_metadata *xlrec = (xl_btree_metadata *) rec;
+				xl_btree_metadata *xlrec;
 
+				xlrec = (xl_btree_metadata *) XLogRecGetBlockData(record, 0,
+																  NULL);
 				appendStringInfo(buf, "oldest_btpo_xact %u; last_cleanup_num_heap_tuples: %f",
 								 xlrec->oldest_btpo_xact,
 								 xlrec->last_cleanup_num_heap_tuples);