kernel-netlink: Implement passthrough type routes and use them on Linux

Enables us to ignore any future kernel features for routes unless
we actually need to consider them for the source IP routes.

Also enables us to actually really skip IPsec processing for those networks
(because even the routes don't touch those packets). It's more what
users expect.

Co-authored-by: Tobias Brunner <[email protected]>

src/libcharon/kernel/kernel_interface.h

@@ -375,12 +375,13 @@ struct kernel_interface_t {
 	 * @param gateway		gateway for this route
 	 * @param src_ip		source ip of the route
 	 * @param if_name		name of the interface the route is bound to
+	 * @param pass			TRUE if route is installed for passthrough policy
 	 * @return				SUCCESS if operation completed
 	 *						ALREADY_DONE if the route already exists
 	 */
 	status_t (*add_route) (kernel_interface_t *this, chunk_t dst_net,
 						   uint8_t prefixlen, host_t *gateway, host_t *src_ip,
-						   char *if_name);
+						   char *if_name, bool pass);
 
 	/**
 	 * Delete a route.
@@ -390,11 +391,12 @@ struct kernel_interface_t {
 	 * @param gateway		gateway for this route
 	 * @param src_ip		source ip of the route
 	 * @param if_name		name of the interface the route is bound to
+	 * @param pass			TRUE if route was installed for passthrough policy
 	 * @return				SUCCESS if operation completed
 	 */
 	status_t (*del_route) (kernel_interface_t *this, chunk_t dst_net,
 						   uint8_t prefixlen, host_t *gateway, host_t *src_ip,
-						   char *if_name);
+						   char *if_name, bool pass);
 
 	/**
 	 * Set up a bypass policy for a given socket.