rbd: move stripe_unit and stripe_count into header

This commit added fetching if fancy striping parameters:
    09186ddb rbd: get and check striping parameters

They are almost unused, but the two fields storing the information
really belonged in the rbd_image_header structure.

This patch moves them there.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>