rbd: kill rbd_image_header->total_snaps

The "total_snaps" field in an rbd header structure is never any
different from the value of "num_snaps" stored within a snapshot
context.  Avoid any confusion by just using the value held within
the snapshot context, and get rid of the "total_snaps" field.

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