dm integrity: Remove extra unlikely helper

In IS_ERR, the unlikely is used for the input parameter,
so these is no need to use it again outside.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>