tproxy: added udp6_lib_lookup function

Just like with IPv4, we need access to the UDP hash table to look up local
sockets, but instead of exporting the global udp_table, export a lookup
function.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: KOVACS Krisztian <hidden@balabit.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>