decnet: Do not build routes to devices without decnet private data.

In particular, make sure we check for decnet private presence
for loopback devices.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/decnet/dn_route.c

@@ -1034,10 +1034,13 @@ static int dn_route_output_slow(struct dst_entry **pprt, const struct flowidn *o
 	if (!fld.daddr) {
 		fld.daddr = fld.saddr;
 
-		err = -EADDRNOTAVAIL;
 		if (dev_out)
 			dev_put(dev_out);
+		err = -EINVAL;
 		dev_out = init_net.loopback_dev;
+		if (!dev_out->dn_ptr)
+			goto out;
+		err = -EADDRNOTAVAIL;
 		dev_hold(dev_out);
 		if (!fld.daddr) {
 			fld.daddr =
@@ -1110,6 +1113,8 @@ static int dn_route_output_slow(struct dst_entry **pprt, const struct flowidn *o
 		if (dev_out == NULL)
 			goto out;
 		dn_db = rcu_dereference_raw(dev_out->dn_ptr);
+		if (!dn_db)
+			goto e_inval;
 		/* Possible improvement - check all devices for local addr */
 		if (dn_dev_islocal(dev_out, fld.daddr)) {
 			dev_put(dev_out);
@@ -1151,6 +1156,8 @@ static int dn_route_output_slow(struct dst_entry **pprt, const struct flowidn *o
 			dev_put(dev_out);
 		dev_out = init_net.loopback_dev;
 		dev_hold(dev_out);
+		if (!dev_out->dn_ptr)
+			goto e_inval;
 		fld.flowidn_oif = dev_out->ifindex;
 		if (res.fi)
 			dn_fib_info_put(res.fi);