The original code did not free the old map.  This patch fixes it.

tj: use @old as memcpy source instead of @chunk->map, and indentation
    and description update
Signed-off-by: NHuang Shijie <shijie8@gmail.com>
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: stable@kernel.org