rbd: num_ops parameter for rbd_osd_req_create()

In preparation for prefixing rbd writes with an allocation hint
introduce a num_ops parameter for rbd_osd_req_create().  The rationale
is that not every write request is a write op that needs to be prefixed
(e.g. watch op), so the num_ops logic needs to be in the callers.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>