scsi: always call notifier on async cancellation

This was found by code inspection.  If the request is cancelled twice,
the notifier is never called on the second cancellation request,
and hence for example a TMF might never finish.

All the calls in scsi_req_cancel_async are idempotent, so the change
is safe.
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <1450290827-30508-2-git-send-email-pbonzini@redhat.com>

hw/scsi/scsi-bus.c

@@ -1759,9 +1759,6 @@ void scsi_req_cancel_async(SCSIRequest *req, Notifier *notifier)
     if (notifier) {
         notifier_list_add(&req->cancel_notifiers, notifier);
     }
-    if (req->io_canceled) {
-        return;
-    }
     scsi_req_ref(req);
     scsi_req_dequeue(req);
     req->io_canceled = true;