Reduce disk bandwidth measurement chunk, 16M should be enough, and avoids spending too much time during calibration

include/starpu_disk.h

@@ -68,7 +68,7 @@ void *starpu_disk_open(unsigned node, void *pos, size_t size);
 
 int starpu_disk_register(struct starpu_disk_ops *func, void *parameter, starpu_ssize_t size);
 
-#define STARPU_DISK_SIZE_MIN (64*1024*1024)
+#define STARPU_DISK_SIZE_MIN (16*1024*1024)
 
 extern int starpu_disk_swap_node;