tcmu: Fix module removal due to stuck unmap_thread thread again

Because the unmap code just after the schdule() returned may take
a long time and if the kthread_stop() is fired just when in this
routine, the module removal maybe stuck too.
Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_user.c

@@ -1707,7 +1707,7 @@ static int unmap_thread_fn(void *data)
 	struct page *page;
 	int i;
 
-	while (1) {
+	while (!kthread_should_stop()) {
 		DEFINE_WAIT(__wait);
 
 		prepare_to_wait(&unmap_wait, &__wait, TASK_INTERRUPTIBLE);