rbd: set image size when header is updated

The image may have been resized.

Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Reviewed-by: Alex Elder <elder@inktank.com>
diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index 9c3a1db..a6bbda2 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -1746,6 +1746,7 @@
 	kfree(rbd_dev->header.snap_names);
 	kfree(rbd_dev->header.snapc);
 
+	rbd_dev->header.image_size = h.image_size;
 	rbd_dev->header.total_snaps = h.total_snaps;
 	rbd_dev->header.snapc = h.snapc;
 	rbd_dev->header.snap_names = h.snap_names;