rx: avoid nat ping during shutdown

rx: avoid nat ping during shutdown

shutdown_rx, unlike rx_Finalize, kills the socket before the conns.
since we call osi_NetSend directly, we lose. just do a simple
check for rxinit_status, and exit immediately before sending if rx
is not up.

Change-Id: Ie34a45a4c1c88fc5732801dcefec89cbc9764f81
Reviewed-on: http://gerrit.openafs.org/5377
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>