mpls: make via address optional for multipath routes

The via address is optional for a single path route, yet is mandatory
when the multipath attribute is used:

  # ip -f mpls route add 100 dev lo
  # ip -f mpls route add 101 nexthop dev lo
  RTNETLINK answers: Invalid argument

Make them consistent by making the via address optional when the
RTA_MULTIPATH attribute is being parsed so that both forms of
specifying the route work.
Signed-off-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mpls/af_mpls.c

@@ -604,10 +604,14 @@ static int mpls_nh_build(struct net *net, struct mpls_route *rt,
 			goto errout;
 	}
 
+	if (via) {
 		err = nla_get_via(via, &nh->nh_via_alen, &nh->nh_via_table,
 				  __mpls_nh_via(rt, nh));
 		if (err)
 			goto errout;
+	} else {
+		nh->nh_via_table = MPLS_NEIGH_TABLE_UNSPEC;
+	}
 
 	err = mpls_nh_assign_dev(net, rt, nh, oif);
 	if (err)
@@ -689,9 +693,6 @@ static int mpls_nh_build_multi(struct mpls_route_config *cfg,
 			nla_newdst = nla_find(attrs, attrlen, RTA_NEWDST);
 		}
 
-		if (!nla_via)
-			goto errout;
-
 		err = mpls_nh_build(cfg->rc_nlinfo.nl_net, rt, nh,
 				    rtnh->rtnh_ifindex, nla_via,
 				    nla_newdst);
@@ -1271,7 +1272,8 @@ static int mpls_dump_route(struct sk_buff *skb, u32 portid, u32 seq, int event,
 							    nh->nh_labels,
 							    nh->nh_label))
 				goto nla_put_failure;
-			if (nla_put_via(skb, nh->nh_via_table,
+			if (nh->nh_via_table != MPLS_NEIGH_TABLE_UNSPEC &&
+			    nla_put_via(skb, nh->nh_via_table,
 					mpls_nh_via(rt, nh),
 					nh->nh_via_alen))
 				goto nla_put_failure;
@@ -1343,6 +1345,8 @@ static inline size_t lfib_nlmsg_size(struct mpls_route *rt)
 
 		for_nexthops(rt) {
 			nhsize += nla_total_size(sizeof(struct rtnexthop));
+			/* RTA_VIA */
+			if (nh->nh_via_table != MPLS_NEIGH_TABLE_UNSPEC)
 				nhsize += nla_total_size(2 + nh->nh_via_alen);
 			if (nh->nh_labels)
 				nhsize += nla_total_size(nh->nh_labels * 4);