rbd: fix snapshot size type

Snapshot sizes should be the same type as regular image sizes. This
only affects their displayed size in sysfs, not the reported size of
an actual block device sizes.

Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Reviewed-by: Alex Elder <elder@dreamhost.com>
Reviewed-by: Yehuda Sadeh <yehuda@hq.newdream.net>