mpls: Return error for RTA_GATEWAY attribute

MPLS does not support nexthops with an MPLS address family.
Specifically, it does not handle RTA_GATEWAY attribute. Make it
clear by returning an error.

Fixes: 03c05665 ("mpls: Netlink commands to add, remove, and dump routes")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mpls/af_mpls.c

@@ -1838,6 +1838,9 @@ static int rtm_to_route_config(struct sk_buff *skb,
 				goto errout;
 			break;
 		}
+		case RTA_GATEWAY:
+			NL_SET_ERR_MSG(extack, "MPLS does not support RTA_GATEWAY attribute");
+			goto errout;
 		case RTA_VIA:
 		{
 			if (nla_get_via(nla, &cfg->rc_via_alen,