z2ram: Small cleanup for z2ram.c

We should use Z2MINOR_COUNT as range argument in blk_unregister_region()

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/block/z2ram.c

@@ -374,7 +374,7 @@ err:
 static void __exit z2_exit(void)
 {
     int i, j;
-    blk_unregister_region(MKDEV(Z2RAM_MAJOR, 0), 256);
+    blk_unregister_region(MKDEV(Z2RAM_MAJOR, 0), Z2MINOR_COUNT);
     unregister_blkdev(Z2RAM_MAJOR, DEVICE_NAME);
     del_gendisk(z2ram_gendisk);
     put_disk(z2ram_gendisk);