tipc: set ub->ifindex for local ipv6 address

Without ub->ifindex set for ipv6 address in tipc_udp_enable(),
ipv6_sock_mc_join() may make the wrong dev join the multicast
address in enable_mcast(). This causes that tipc links would
never be created.

So fix it by getting the right netdev and setting ub->ifindex,
as it does for ipv4 address.

Reported-by: Shuang Li <shuali@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>