[NETNS][IPV6]: inet6_addr - ipv6_chk_same_addr namespace aware

This patch makes ipv6_chk_same_addr function to be aware of the
network namespace. The addresses not belonging to the network
namespace are discarded.

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>