[NETNS]: Add missing initialization of nl_info.nl_net in rtm_to_fib6_config()

Add missing initialization of the new nl_info.nl_net field in
rtm_to_fib6_config(). This will be needed the store network namespace
associated to the fib6_config struct.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>