Add TEST LINK message responses, which also serves as preparation for
support of sockopt TCP_KEEPALIVE.
Signed-off-by: NKarsten Graul <kgraul@linux.vnet.ibm.com>
Signed-off-by: NUrsula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>