rbd: don't set data in rbd_osd_req_format_op()

Currently an object request has its osd request's data field set in
rbd_osd_req_format_op().  That assumes a single osd op per object
request, and that won't be the case for long.

Move the code that sets this out and into the caller.

Rename rbd_osd_req_format_op() to be just rbd_osd_req_format(),
removing the notion that it's doing anything op-specific.

This and the next patch resolve:
    http://tracker.ceph.com/issues/4658



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