tipc: refactor function tipc_sk_timeout()

We refactor this function as a preparation for the coming commits in
the same series.

Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>