dm integrity: Fix UAF in dm_integrity_dtr()

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

Therefore, cancelling timer again in dm_integrity_dtr().

Cc: stable@vger.kernel.org
Fixes: 7eada909

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