[NETNS][IPV6] ip6_fib - clean node use namespace

The fib6_clean_node function should have the network namespace it is
working on. The fib6_cleaner_t structure is extended with the network
namespace field to be passed to the fib6_clean_node function.

The different functions calling the fib6_clean_node function are
extended with the netns parameter when needed to propagate the netns
pointer.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>