- Ensure that we reset req->rq_received before resending if the
     server has sent us a garbage reply.

   - Whenever we grab the xprt_lock_write "semaphore" ensure that we
     reset req->rq_bytes_sent.

   - When resending a TCP request, do not interrupt in the
     middle of transmission even if we do get a reply from the server.

   - Protect the call to rpc_calc_rto() against modification while
     we are calculating