nfs: allow the v4.1 callback thread to freeze

The v4.1 callback thread has set_freezable() at the top, but it doesn't
ever try to freeze within the loop. Have it call try_to_freeze() at the
top of the loop. If a freeze event occurs, recheck kthread_should_stop()
after thawing.
Reported-by: Yanchuan Nian <ycnian@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/callback.c

@@ -125,6 +125,9 @@ nfs41_callback_svc(void *vrqstp)
 	set_freezable();
 
 	while (!kthread_should_stop()) {
+		if (try_to_freeze())
+			continue;
+
 		prepare_to_wait(&serv->sv_cb_waitq, &wq, TASK_INTERRUPTIBLE);
 		spin_lock_bh(&serv->sv_cb_lock);
 		if (!list_empty(&serv->sv_cb_list)) {