nspawn: explicitly remove veth links after use (#3111)

* sd-netlink: permit RTM_DELLINK messages with no ifindex

This is useful for removing network interfaces by name.

* nspawn: explicitly remove veth links we created after use

Sometimes the kernel keeps veth links pinned after the namespace they have been
joined to died. Let's hence explicitly remove veth links after use.

Fixes: #2173

src/nspawn/nspawn.c

@@ -3713,6 +3713,7 @@ int main(int argc, char *argv[]) {
                 }
 
                 expose_port_flush(arg_expose_ports, &exposed);
+                (void) remove_veth_links(veth_name, arg_network_veth_extra);
         }
 
 finish:
@@ -3745,6 +3746,7 @@ finish:
         }
 
         expose_port_flush(arg_expose_ports, &exposed);
+        (void) remove_veth_links(veth_name, arg_network_veth_extra);
 
         free(arg_directory);
         free(arg_template);