kernel-interface: Add destination prefix to get_nexthop()

This allows to determine the next hop to reach a subnet, for instance, when
installing routes for shunt policies.

src/libhydra/plugins/kernel_netlink/kernel_netlink_ipsec.c

@@ -2137,7 +2137,7 @@ static status_t add_policy_internal(private_kernel_netlink_ipsec_t *this,
 			/* get the nexthop to src (src as we are in POLICY_FWD) */
 			route->gateway = hydra->kernel_interface->get_nexthop(
 											hydra->kernel_interface, ipsec->src,
-											ipsec->dst);
+											-1, ipsec->dst);
 			route->dst_net = chunk_alloc(policy->sel.family == AF_INET ? 4 : 16);
 			memcpy(route->dst_net.ptr, &policy->sel.saddr, route->dst_net.len);
 

src/libhydra/plugins/kernel_netlink/kernel_netlink_net.c

@@ -1818,7 +1818,7 @@ METHOD(kernel_net_t, get_source_addr, host_t*,
 }
 
 METHOD(kernel_net_t, get_nexthop, host_t*,
-	private_kernel_netlink_net_t *this, host_t *dest, host_t *src)
+	private_kernel_netlink_net_t *this, host_t *dest, int prefix, host_t *src)
 {
 	return get_route(this, dest, TRUE, src, 0);
 }