correct inaccurate comment regarding zone_table's usage

The comment describing the usage of zone_table[] assumes the existence of an unsigned char page->zone field from the original implementation of page->zone size reduction. This patch corrects the comment to accurately describe the lookup mechanism used by page_zone() and also to mention explicitly the sole user of the table, page_zone().

Against 2.5.19.

Cheers,
Bill

===== mm/page_alloc.c 1.64 vs edited =====

@@ -31,7 +31,10 @@
 LIST_HEAD(inactive_list);
 pg_data_t *pgdat_list;
 
-/* Used to look up the address of the struct zone encoded in page->zone */
+/*
+ * Used by page_zone() to look up the address of the struct zone whose
+ * id is encoded in the upper bits of page->flags
+ */
 zone_t *zone_table[MAX_NR_ZONES*MAX_NR_NODES];
 EXPORT_SYMBOL(zone_table);