[NETFILTER]: nf_conntrack_netlink: sync expectation dumping with conntrack table dumping

Resync expectation table dumping code with conntrack dumping: don't
rely on the unique ID anymore since that requires to walk the list
backwards, which doesn't work with the upcoming conversion to hlists.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>