rbd: only set device exists flag when ready

Hold off setting the EXISTS rbd device flag until just before we
announce the disk as available for use.  There's no point in doing
so any earlier than that, and at that point the device truly is
fully set up and ready to use.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -881,7 +881,6 @@ static int rbd_dev_set_mapping(struct rbd_device *rbd_dev)
 		rbd_dev->mapping.features = snap->features;
 		rbd_dev->mapping.read_only = true;
 	}
-	set_bit(RBD_DEV_FLAG_EXISTS, &rbd_dev->flags);
 
 	return 0;
 }
@@ -4785,6 +4784,7 @@ static int rbd_dev_probe_finish(struct rbd_device *rbd_dev)
 
 	/* Everything's ready.  Announce the disk to the world. */
 
+	set_bit(RBD_DEV_FLAG_EXISTS, &rbd_dev->flags);
 	add_disk(rbd_dev->disk);
 
 	pr_info("%s: added with size 0x%llx\n", rbd_dev->disk->disk_name,