rbd: update remaining header fields for v2
There are three fields that are not yet updated for format 2 rbd
image headers: the version of the header object; the encryption
type; and the compression type. There is no interface defined for
fetching the latter two, so just initialize them explicitly to 0 for
now.
Change rbd_dev_v2_snap_context() so the caller can be supplied the
version for the header object.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>