tcmu: fix module removal due to stuck thread

We need to do a kthread_should_stop to check when kthread_stop has been
called.

This was a regression added in

b6df4b79
tcmu: Add global data block pool support

so not sure if you wanted to merge it in with that patch or what.
Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_user.c

@@ -1560,6 +1560,9 @@ static int unmap_thread_fn(void *data)
 		schedule();
 		finish_wait(&unmap_wait, &__wait);
 
+		if (kthread_should_stop())
+			break;
+
 		mutex_lock(&root_udev_mutex);
 		list_for_each_entry(udev, &root_udev, node) {
 			mutex_lock(&udev->cmdr_lock);