[NETNS][IPV6]: inet6_addr - ipv6_get_ifaddr namespace aware

The inet6_addr_lst is browsed taking into account the network
namespace specified as parameter. If an address does not belong
to the specified namespace, it is ignored.

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>