tipc: fix socket list regression in new nl api

Commit 07f6c4bc

 (tipc: convert tipc reference table to use generic
rhashtable) introduced a problem with port listing in the new netlink
API. It broke the resume functionality resulting in a never ending
loop. This was caused by starting with the first hash table every time
subsequently never returning an empty skb (terminating).

This patch fixes the resume mechanism by keeping a logical reference
to the last hash table along with a logical reference to the socket
(port) that didn't fit in the previous message.

Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>