rbd: don't return 0 on unmap if RBD_DEV_FLAG_REMOVING is set

There is a window between when RBD_DEV_FLAG_REMOVING is set and when
the device is removed from rbd_dev_list.  During this window, we set
"already" and return 0.

Returning 0 from write(2) can confuse userspace tools because
0 indicates that nothing was written.  In particular, "rbd unmap"
will retry the write multiple times a second:

  10:28:05.463299 write(4, "0", 1)        = 0
  10:28:05.463509 write(4, "0", 1)        = 0
  10:28:05.463720 write(4, "0", 1)        = 0
  10:28:05.463942 write(4, "0", 1)        = 0
  10:28:05.464155 write(4, "0", 1)        = 0

Cc: stable@vger.kernel.org
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Tested-by: NDongsheng Yang <dongsheng.yang@easystack.cn>

drivers/block/rbd.c

@@ -5986,7 +5986,6 @@ static ssize_t do_rbd_remove(struct bus_type *bus,
 	struct list_head *tmp;
 	int dev_id;
 	char opt_buf[6];
-	bool already = false;
 	bool force = false;
 	int ret;
 
@@ -6019,13 +6018,13 @@ static ssize_t do_rbd_remove(struct bus_type *bus,
 		spin_lock_irq(&rbd_dev->lock);
 		if (rbd_dev->open_count && !force)
 			ret = -EBUSY;
-		else
-			already = test_and_set_bit(RBD_DEV_FLAG_REMOVING,
-							&rbd_dev->flags);
+		else if (test_and_set_bit(RBD_DEV_FLAG_REMOVING,
+					  &rbd_dev->flags))
+			ret = -EINPROGRESS;
 		spin_unlock_irq(&rbd_dev->lock);
 	}
 	spin_unlock(&rbd_dev_list_lock);
-	if (ret < 0 || already)
+	if (ret)
 		return ret;
 
 	if (force) {