rbd: standardize ceph_osd_request variable names

There are spots where a ceph_osds_request pointer variable is given
the name "req".  Since we're dealing with (at least) three types of
requests (block layer, rbd, and osd), I find this slightly
distracting.

Change such instances to use "osd_req" consistently to make the
abstraction represented a little more obvious.

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