fix alloc_bootmem_low_pages

diff -Nru a/mm/bootmem.c b/mm/bootmem.c
@@ -48,8 +48,24 @@
 	bootmem_data_t *bdata = pgdat->bdata;
 	unsigned long mapsize = ((end - start)+7)/8;
 
-	pgdat->pgdat_next = pgdat_list;
-	pgdat_list = pgdat;
+
+	/*
+	 * sort pgdat_list so that the lowest one comes first,
+	 * which makes alloc_bootmem_low_pages work as desired.
+	 */
+	if (!pgdat_list || pgdat_list->node_start_pfn > pgdat->node_start_pfn) {
+		pgdat->pgdat_next = pgdat_list;
+		pgdat_list = pgdat;
+	} else {
+		pg_data_t *tmp = pgdat_list;
+		while (tmp->pgdat_next) {
+			if (tmp->pgdat_next->node_start_pfn > pgdat->node_start_pfn)
+				break;
+			tmp = tmp->pgdat_next;
+		}
+		pgdat->pgdat_next = tmp->pgdat_next;
+		tmp->pgdat_next = pgdat;
+	}
 
 	mapsize = (mapsize + (sizeof(long) - 1UL)) & ~(sizeof(long) - 1UL);
 	bdata->node_bootmem_map = phys_to_virt(mapstart << PAGE_SHIFT);