[PATCH] Set a minimum hash table size for wait_on_page()

Fixes the problem identified by Miles Bader on extremely small zones:
calling hash_long with `bits = 0' is treated as `bits = 32'.

So don't permit the zone to have a one-slot waitqueue hashtable.

mm/page_alloc.c

@@ -996,7 +996,7 @@ static inline unsigned long wait_table_size(unsigned long pages)
 	 */
 	size = min(size, 4096UL);
 
-	return size;
+	return max(size, 4UL);
 }
 
 /*