rbd: protect read of snapshot sequence number

(cherry picked from commit 403f24d3)

This is updated whenever a snapshot is added or deleted, and the
snapc pointer is changed with every refresh of the header.
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Reviewed-by: Alex Elder <elder@dreamhost.com>
Reviewed-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/block/rbd.c

@@ -1683,7 +1683,9 @@ static int rbd_header_add_snap(struct rbd_device *dev,
 	if (ret < 0)
 		return ret;
 
+	down_write(&dev->header_rwsem);
 	dev->header.snapc->seq = new_snapid;
+	up_write(&dev->header_rwsem);
 
 	return 0;
 bad: