dm rq: clear kworker_task if kthread_run() returned an error

cleanup_mapped_device() calls kthread_stop() if kworker_task is
non-NULL.  Currently the assigned value could be a valid task struct or
an error code (e.g -ENOMEM).  Reset md->kworker_task to NULL if
kthread_run() returned an erorr.

Fixes: 7193a9de ("dm rq: check kthread_run return for .request_fn request-based DM")
Cc: stable@vger.kernel.org # 4.8
Reported-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>