- Fix the check for whether or not the received message length has
   somehow been truncated: we need to use req->rq_received rather
   than the receive buffer length (req->rq_rlen).

 - Ensure that we set res->eof correctly. In particular, we need to
   clear it if we find ourselves attempting to recover from a
   truncated READ.

 - Don't set PageUptodate() on those pages that are the victim of
   message truncation.