dm ioctl: Avoid pointer arithmetic overflow

Especially on 32-bit systems, it is possible for the pointer
arithmetic to overflow and cause a userspace pointer to be
dereferenced in the kernel.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>