[IP6TUNNEL]: Make tunnels hashes per-net.

Move hashes in the struct ip6_tnl_net, replace tnls_xxx[] with 
ip6n->tnlx_xxx[] and handle init and exit appropriately.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>