[NETNS][IPV6] ip6_fib - fib6_clean_all handle several network namespaces

The function fib6_clean_all takes the network namespace as
parameter. That allows to flush the routes related to a specific
network namespace.

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