[PATCH] Fix up compiler inefficiencies

Oh, bugger...

The recent mmzone.h change triggered some very, very ugly things in
next_zone(), is_highmem(), is_normal() and zone_idx() because it changed
the size of "struct zone" subtly.

Pointer subtraction is no fun when sizeof of object gets weird and poor
gcc blows its brains out trying to optimize that...  This slowed down an
ARM cross-build by a factor of 2.5. 

Turn the pointer subtraction tests into pointer addition tests instead
where possible, making them trivial to deal with (and gets better code,
in addition to faster compile).

zone_idx() still does a subtraction.

include/linux/mmzone.h

@@ -311,7 +311,7 @@ static inline struct zone *next_zone(struct zone *zone)
 {
 	pg_data_t *pgdat = zone->zone_pgdat;
 
-	if (zone - pgdat->node_zones < MAX_NR_ZONES - 1)
+	if (zone < pgdat->node_zones + MAX_NR_ZONES - 1)
 		zone++;
 	else if (pgdat->pgdat_next) {
 		pgdat = pgdat->pgdat_next;
@@ -357,12 +357,12 @@ static inline int is_normal_idx(int idx)
  */
 static inline int is_highmem(struct zone *zone)
 {
-	return (is_highmem_idx(zone - zone->zone_pgdat->node_zones));
+	return zone == zone->zone_pgdat->node_zones + ZONE_HIGHMEM;
 }
 
 static inline int is_normal(struct zone *zone)
 {
-	return (is_normal_idx(zone - zone->zone_pgdat->node_zones));
+	return zone == zone->zone_pgdat->node_zones + ZONE_NORMAL;
 }
 
 /* These two functions are used to setup the per zone pages min values */