rbd: separate initialization of osd data

The osd data for a request is currently initialized inside
rbd_osd_req_create(), but that assumes an object request's data
belongs in the osd request's data in or data out field.

There are only three places where requests with data are set up, and
it turns out it's easier to call just the osd data init routines
directly there rather than handling it in rbd_osd_req_create().

(The real motivation here is moving toward getting rid of the
osd request in and out data fields.)

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