[IPV6]: Add ifindex field to ND user option messages.

Userland neighbor discovery options are typically heavily involved with
the interface on which thay are received: add a missing ifindex field to
the original struct. Thanks to Rémi Denis-Courmont.

Signed-off-by: Pierre Ynard <linkfanel@yahoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>