Fix typo in freeze_table_age implementation

The original code used freeze_min_age instead of freeze_table_age.  The
main consequence of this mistake is that lowering freeze_min_age would
cause full-table scans to occur much more frequently, which causes
serious issues because the number of writes required is much larger.
That feature (freeze_min_age) is supposed to affect only how soon tuples
are frozen; some pages should still be skipped due to the visibility
map.

Backpatch to 8.4, where the freeze_table_age feature was introduced.

Report and patch from Andres Freund

Author: alvherre

src/backend/commands/vacuum.c

@@ -449,7 +449,7 @@ vacuum_set_xid_limits(int freeze_min_age,
 		 * VACUUM schedule, the nightly VACUUM gets a chance to freeze tuples
 		 * before anti-wraparound autovacuum is launched.
 		 */
-		freezetable = freeze_min_age;
+		freezetable = freeze_table_age;
 		if (freezetable < 0)
 			freezetable = vacuum_freeze_table_age;
 		freezetable = Min(freezetable, autovacuum_freeze_max_age * 0.95);