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>
(cherry picked from commit 93a24e084d67ba2fcb9a4c289135825b623ec864)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>