rbd: end request on error in rbd_do_request() caller

Only one of the three callers of rbd_do_request() provide a
collection structure to aggregate status.

If an error occurs in rbd_do_request(), have the caller
take care of calling rbd_coll_end_req() if necessary in
that one spot.

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