dm: Make use of __assign_bit() API

We have for some time the __assign_bit() API to replace
open coded
    if (foo)
        __set_bit(n, bar);
    else
        __clear_bit(n, bar);

Use this API to simplify the code. No functional change
intended.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>