zebra: update kernel routes with interface check

Zebra NH check is wrong -- it does not update missing interface routes

Steps to reproduce:
```
ip link set dev eth0 up
ip -4 addr add 192.168.0.2/24 dev eth0
ip -4 route add default via 192.168.0.1
ip -4 addr flush dev eth0
```

Before
```
K>* 0.0.0.0/0 [0/0] via 192.168.0.1, eth0, 00:00:07
```

After
Route is removed from zebra

Signed-off-by: Yaroslav Kholod <[email protected]>

zebra/zebra_nhg.c

@@ -2645,10 +2645,12 @@ static unsigned nexthop_active_check(struct route_node *rn,
 	 */
 	if (re->type == ZEBRA_ROUTE_KERNEL || re->type == ZEBRA_ROUTE_SYSTEM) {
 		struct interface *ifp;
+		int connected_if_count = 0;
 
 		ifp = if_lookup_by_index(nexthop->ifindex, nexthop->vrf_id);
+		connected_if_count = connected_count_by_family(ifp->connected, AF_INET) + connected_count_by_family(ifp->connected, AF_INET6);
 
-		if (ifp && ifp->vrf->vrf_id == vrf_id && if_is_up(ifp)) {
+		if (ifp && ifp->vrf->vrf_id == vrf_id && if_is_up(ifp) && connected_if_count) {
 			SET_FLAG(nexthop->flags, NEXTHOP_FLAG_ACTIVE);
 			goto skip_check;
 		}