netfilter: nf_conntrack: fix rt6i_gateway checks for H.323 helper

Now when rt6_nexthop() can return nexthop address we can use it
for proper nexthop comparison of directly connected destinations.
For more information refer to commit bbb5823cf742a7
("netfilter: nf_conntrack: fix rt_gateway checks for H.323 helper").

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>