percpu: fix a memory leak in pcpu_extend_area_map()

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: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@kernel.org