We need this functionality for async OSC RPC to support LU-884.

This is something like fake req, but it should be more well encapsulated. The interfaces would be like:

1. void *ptlrpc_allocate_work(struct obd_import *imp, int (*cb)(const struct lu_env *, void *), void *cbdata);
Initialize a callback data; handler will store a ptlrpc_request but caller shouldn't know this detail;

2. int ptlrpc_run_work(void *handler);
run the callback in ptlrpcd context if the request(handler) is NOT already in ptlrpcd list;

3. void ptlrpc_destroy_work(void *handler);