[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 RĂ©mi Denis-Courmont.

Signed-off-by: Pierre Ynard <linkfanel@yahoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 36f7dbf..67997a7 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -1037,6 +1037,7 @@
 
 	ndmsg = nlmsg_data(nlh);
 	ndmsg->nduseropt_family = AF_INET6;
+	ndmsg->nduseropt_ifindex = ra->dev->ifindex;
 	ndmsg->nduseropt_icmp_type = icmp6h->icmp6_type;
 	ndmsg->nduseropt_icmp_code = icmp6h->icmp6_code;
 	ndmsg->nduseropt_opts_len = opt->nd_opt_len << 3;