Fix ValidateExpectedRoute with non default routes and nil GW

Using ptp plugin with non default routes, we get the following error
when cri-o call CheckNetworkList():
```
Expected Route {Dst:{IP:198.18.128.0 Mask:ffff8000} GW:<nil>} not found in routing table
```
Using cniVersion 0.3.1 to bypass the check, we can see that the
route is added with a gateway
```
$ ip r
198.18.0.0/17 via 198.18.0.1 dev eth0 src 198.18.3.102
198.18.0.1 dev eth0 scope link src 198.18.3.102
198.18.128.0/17 via 198.18.0.1 dev eth0
```

If GW is nil only check if we have a route with a DST that matches, and
ignore the GW.

Fixes #886
Signed-off-by: Etienne Champetier <e.champetier@ateme.com>

pkg/ip/utils_linux.go

@@ -83,7 +83,10 @@ func ValidateExpectedRoute(resultRoutes []*types.Route) error {
 	// Ensure that each static route in prevResults is found in the routing table
 	for _, route := range resultRoutes {
 		find := &netlink.Route{Dst: &route.Dst, Gw: route.GW}
-		routeFilter := netlink.RT_FILTER_DST | netlink.RT_FILTER_GW
+		routeFilter := netlink.RT_FILTER_DST
+		if route.GW != nil {
+			routeFilter |= netlink.RT_FILTER_GW
+		}
 		var family int
 
 		switch {