[NETNS][IPV6]: inet6_addr - check ipv6 address per namespace

When a new address is added, we must check if the new address does not
already exists.  This patch makes this check to be aware of a network
namespace, so the check will look if the address already exists for
the specified network namespace. While the addresses are browsed, the
addresses which do not belong to the 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>