mirror_drained_poll() returns true whenever the job is cancelled,
because "we [can] be sure that it won't issue more requests".  However,
this is only true for force-cancelled jobs, so use job_is_cancelled().

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211006151940.214590-10-hreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>