The passed-in header structure is zeroed in rbd_header_from_disk().
Instead, have the caller do it. Note that there are two callers,
rbd_dev_v1_refresh() and rbd_dev_v1_probe(). The latter already has
a zeroed header structure so zeroing it isn't necessary there.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>