[NETNS][IPV6]: inet6_addr - make ipv6_chk_home_addr namespace aware

Looks if the address is belonging to the network namespace, otherwise
discard the address for the check.

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>