Revert 56-bit relfilenode change and follow-up commits.

There are still some alignment-related failures in the buildfarm,
which might or might not be able to be fixed quickly, but I've also
just realized that it increased the size of many WAL records by 4 bytes
because a block reference contains a RelFileLocator. The effect of that
hasn't been studied or discussed, so revert for now.

src/test/regress/sql/alter_table.sql

@@ -1478,8 +1478,9 @@ select relname,
   c.oid = oldoid as orig_oid,
   case relfilenode
     when 0 then 'none'
+    when c.oid then 'own'
     when oldfilenode then 'orig'
-    else 'new'
+    else 'OTHER'
     end as storage,
   obj_description(c.oid, 'pg_class') as desc
   from pg_class c left join old_oids using (relname)
@@ -1498,8 +1499,9 @@ select relname,
   c.oid = oldoid as orig_oid,
   case relfilenode
     when 0 then 'none'
+    when c.oid then 'own'
     when oldfilenode then 'orig'
-    else 'new'
+    else 'OTHER'
     end as storage,
   obj_description(c.oid, 'pg_class') as desc
   from pg_class c left join old_oids using (relname)
@@ -1639,7 +1641,7 @@ CREATE FUNCTION check_ddl_rewrite(p_tablename regclass, p_ddl text)
 RETURNS boolean
 LANGUAGE plpgsql AS $$
 DECLARE
-    v_relfilenode int8;
+    v_relfilenode oid;
 BEGIN
     v_relfilenode := relfilenode FROM pg_class WHERE oid = p_tablename;