[NETNS][IPV6] ndisc - make ndisc handle multiple network namespaces

Make ndisc handle multiple network namespaces:
Remove references to init_net, add network namespace parameters and add
pernet_operations for ndisc

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>