layers/inet6/routes: fix ndp resolution without default route

Sending a IPv6 packet without Ether.dst mac defined fails when there is
not default IPv6 route defined with the following message:

WARNING: No route found for IPv6 destination fd00:200::1 (no default route?)

Because, when there is no default IPv6 route defined, conf.iface6 is
initialized to the loopback interface and the conf.route6.route() method
is called with dev=conf.iface6.

According to the docstring of conf.route6.route():

  if optional 'dev' parameter is provided a specific interface,
  filtering is performed to limit search to route associated to
  that interface.

This will result in the system never finding on which interface it
should send a NDP packet.

Do not call conf.route6.route() with dev=conf.iface6.

Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>
Signed-off-by: Robin Jarry <robin.jarry@6wind.com>