client: fix access violation

client: fix access violation

The MetaRequest is a refcounted object. On construction, it will default ref to 1.
The general process to make an MDS request will be:

  MetaRequest *req = new MetaRequest(op);
  // fill in something
  int r = make_request(req, uid, gid, target);
  return r;

As mentioned, make_request() will put the passed request before returning(and thereby
delete the request automatically).

To be more specific here, the make_request() will call "mds_requests[tid] = request->get();"
to keep the request well local tracked on entry(which raises the ref of request to 2).
And on error exit(especially for the following no reply is received case), by calling both

    unregister_request(request);
    put_request(request); // ours

, it will decrease the reference of the corresponding request to zero and thereby delete request.
Therefore the call to request thereafter is at risk of "access violation".

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>