dm cache: Fix UAF in destroy()

Dm_cache also has the same UAF problem when dm_resume()
and dm_destroy() are concurrent.

Therefore, cancelling timer again in destroy().

Cc: stable@vger.kernel.org
Fixes: c6b4fcba

 ("dm: add cache target")
Signed-off-by: Luo Meng <luomeng12@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>