Fix erroneous replay of GIN_UPDATE_META_PAGE WAL records.

A simple thinko in ginRedoUpdateMetapage, namely failing to increment a
loop counter, led to inserting records into the last pending-list page in
the wrong order (the opposite of that intended).  So far as I can tell,
this would not upset the code that eventually flushes pending items into
the main part of the GIN index.  But it did break the code that searched
the pending list for matches, resulting in transient failure to find
matching entries during index lookups, as illustrated in bug #6307 from
Maksym Boguk.

Back-patch to 8.4 where the incorrect code was introduced.

src/backend/access/gin/ginxlog.c

@@ -528,6 +528,8 @@ ginRedoUpdateMetapage(XLogRecPtr lsn, XLogRecord *record)
 							elog(ERROR, "failed to add item to index page");
 
 						tuples = (IndexTuple) (((char *) tuples) + tupsize);
+
+						off++;
 					}
 
 					/*