9pnet_rdma: add cancelled()

Take into account posted recv buffers that will never receive their
reply.

The RDMA code posts a recv buffer for each request that it sends.
When a request is flushed, it is possible that this request will
never receive a reply, and that one recv buffer will stay unused on
the recv queue.

It is then possible, if this scenario happens several times, to have the
recv queue full, and have the 9pnet_rmda module unable to send new requests.

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