kswapd and MM overload fix

[Andrea Arcangeli]

Anybody running on a machine with some zone empty (<16Mbyte boxes (PDAs),
<1G x86 with highmem enabled kernel or an arch with an iommu like alpha)
probably noticed that the MM was unusable on those hardware
configurations (as I also incidentally mentioned a few times on l-k in
the last months).

Yesterday I checked and here it is the fix (included in 2.4.5aa3).

--- 2.4.5aa3/mm/vmscan.c.~1~	Sat May 26 04:03:50 2001
+++ 2.4.5aa3/mm/vmscan.c	Tue Jun  5 03:41:06 2001
@@ -791,6 +788,8 @@
 		for(i = 0; i < MAX_NR_ZONES; i++) {
 			int zone_shortage;
 			zone_t *zone = pgdat->node_zones+ i;
+			if (!zone->size)
+				continue;
 
 			zone_shortage = zone->pages_high;
 			zone_shortage -= zone->inactive_dirty_pages;

Andrea