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/libcharon/plugins/kernel_iph/kernel_iph_net.c

@@ -562,7 +562,7 @@ METHOD(kernel_net_t, get_source_addr, host_t*,
 }
 
 METHOD(kernel_net_t, get_nexthop, host_t*,
-	private_kernel_iph_net_t *this, host_t *dest, host_t *src)
+	private_kernel_iph_net_t *this, host_t *dest, int prefix, host_t *src)
 {
 	MIB_IPFORWARD_ROW2 route;
 	SOCKADDR_INET best, *sai_dst, *sai_src = NULL;

src/libcharon/plugins/kernel_libipsec/kernel_libipsec_ipsec.c

@@ -314,7 +314,7 @@ static void add_exclude_route(private_kernel_libipsec_ipsec_t *this,
 	{
 		DBG2(DBG_KNL, "installing new exclude route for %H src %H", dst, src);
 		gtw = hydra->kernel_interface->get_nexthop(hydra->kernel_interface,
-												   dst, NULL);
+												   dst, -1, NULL);
 		if (gtw)
 		{
 			char *if_name = NULL;
@@ -445,7 +445,7 @@ static bool install_route(private_kernel_libipsec_ipsec_t *this,
 #ifndef __linux__
 	/* on Linux we cant't install a gateway */
 	route->gateway = hydra->kernel_interface->get_nexthop(
-											hydra->kernel_interface, dst, src);
+										hydra->kernel_interface, dst, -1, src);
 #endif
 
 	if (policy->route)

src/libcharon/plugins/kernel_wfp/kernel_wfp_ipsec.c

@@ -1348,7 +1348,7 @@ static bool manage_route(private_kernel_wfp_ipsec_t *this,
 		return FALSE;
 	}
 	gtw = hydra->kernel_interface->get_nexthop(hydra->kernel_interface,
-											   remote, local);
+											   remote, -1, local);
 	if (add)
 	{
 		done = install_route(this, dst, mask, src, gtw);