mm/highmem: simplify is_highmem()

is_highmem() can be simplified by use of is_highmem_idx().  This patch
removes redundant code and will make it easier to maintain if the zone
policy is changed or a new zone is added.

(akpm: saves me 25 bytes of text per is_highmem() callsite)
Signed-off-by: Chanho Min <chanho.min@lge.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -828,10 +828,7 @@ static inline int is_highmem_idx(enum zone_type idx)
 static inline int is_highmem(struct zone *zone)
 {
 #ifdef CONFIG_HIGHMEM
-	int zone_off = (char *)zone - (char *)zone->zone_pgdat->node_zones;
-	return zone_off == ZONE_HIGHMEM * sizeof(*zone) ||
-	       (zone_off == ZONE_MOVABLE * sizeof(*zone) &&
-		zone_movable_is_highmem());
+	return is_highmem_idx(zone_idx(zone));
 #else
 	return 0;
 #endif