Patch from Robert Love.

Attached patch implements for_each_zone(zont_t *) which is a helper
macro to cleanup code of the form:

        for (pgdat = pgdat_list; pgdat; pgdat = pgdat->node_next) {
                for (i = 0; i < MAX_NR_ZONES; ++i) {
                        zone_t * z = pgdat->node_zones + i;
                        /* ... */
                }
        }

and replace it with:

        for_each_zone(zone) {
                /* ... */
        }

This patch only replaces one use of the above loop with the new macro.
Pending code, however, currently in the full rmap patch uses
for_each_zone more extensively.