9P/RDMA: Do not free req->rc in error handling in rdma_request()

rdma_request() should never be in charge of freeing rc.

When an error occurs:
* Either the rc buffer has been recv_post()'ed.
  then kfree()'ing it certainly is a bad idea.
* Or is has not, and in that case req->rc still points to it,
  hence it needs not be freed.

Signed-off-by: Simon Derr <simon.derr@bull.net>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>