[IPV6]: Add ndisc_netdev_notifier unregister.

If inet6_init() fails later than ndisc_init() call, or IPv6 module is
unloaded, ndisc_netdev_notifier call remains in the list and will follows in
oops later.

Signed-off-by: Dmitry Mishin <dim@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>