xvif(4): Omit needless membars in xennetback_rx_copy_process.

- No need for barrier around touching req_cons and rsp_prod_pvt,
  which are private.

- RING_PUSH_RESPONSES_AND_CHECK_NOTIFY updates the shared req_prod and
  then issues xen_mb, which is all that we need between the update of
  shared req_prod and hypervisor_notify_via_evtchn.

  (Between updating the shared req_prod and issuing
  hypervisor_notify_via_evtchn, only xen_wmb is needed.  But after
  writing to the shared req_prod, RING_PUSH_REQUESTS_AND_CHECK_NOTIFY
  must also read from the shared rsp_event, which requires the
  store-before-load ordering that only xen_mb provides.)

(riastradh)