net/smc: periodic testlink support

Add periodic LLC testlink support to ensure the link is still active.
The interval time is initialized using the value of
sysctl_tcp_keepalive_time.

Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>