rbd: standardize rbd_request variable names

There are two names used for items of rbd_request structure type:
"req" and "req_data".  The former name is also used to represent
items of pointers to struct ceph_osd_request.

Change all variables that have these names so they are instead
called "rbd_req" consistently.

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