net/ipv4: Add extack message that dev is required for ONLINK

Make IPv4 consistent with IPv6 and return an extack message that the
ONLINK flag requires a nexthop device.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_semantics.c

@@ -797,8 +797,10 @@ static int fib_check_nh(struct fib_config *cfg, struct fib_nh *nh,
 				return -EINVAL;
 			}
 			dev = __dev_get_by_index(net, nh->nh_oif);
-			if (!dev)
+			if (!dev) {
+				NL_SET_ERR_MSG(extack, "Nexthop device required for onlink");
 				return -ENODEV;
+			}
 			if (!(dev->flags & IFF_UP)) {
 				NL_SET_ERR_MSG(extack,
 					       "Nexthop device is not up");