[PATCH] Fix i386 bootup with HIGHMEM+SLAB_DEBUG+NUMA and no real

For some reason I booted a NUMA and SLAB_DEBUG i386 kernel on a non
NUMA 512MB machine.  This caused an oops at bootup in change_page_attr.
The reason was that highmem_start_start page ended up zero and 
that triggered the highmem check in change_page_attr when the
slab debug code would unmap a kernel mapping.

Fix is straightforward: if there is no highmem set highmem_start_page
to max_low_pfn+1

arch/i386/mm/discontig.c

@@ -448,7 +448,11 @@ void __init set_highmem_pages_init(int bad_ppro)
 void __init set_max_mapnr_init(void)
 {
 #ifdef CONFIG_HIGHMEM
-	highmem_start_page = NODE_DATA(0)->node_zones[ZONE_HIGHMEM].zone_mem_map;
+	struct zone *high0 = &NODE_DATA(0)->node_zones[ZONE_HIGHMEM];
+	if (high0->spanned_pages > 0)
+	      	highmem_start_page = high0->zone_mem_map;
+	else
+		highmem_start_page = pfn_to_page(max_low_pfn+1); 
 	num_physpages = highend_pfn;
 #else
 	num_physpages = max_low_pfn;