tipc: refactor function tipc_sk_filter_connect()

We refactor the function tipc_sk_filter_connect(), both to make it
more readable and as a preparation for the next commit.

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>