network/route: make the route section invalid when an invalid MTUBytes= is specified

We usually set the invalid flag for a section if a setting in the section has
an invalid value. Let's also do the same thing for MTUBytes= in [Route].

src/network/networkd-route.c

@@ -2814,7 +2814,7 @@ int config_parse_route_mtu(
         }
 
         r = config_parse_mtu(unit, filename, line, section, section_line, lvalue, ltype, rvalue, &n->mtu, userdata);
-        if (r < 0)
+        if (r <= 0)
                 return r;
 
         TAKE_PTR(n);

src/shared/conf-parser.c

@@ -1592,7 +1592,7 @@ int config_parse_mtu(
                 return 0;
         }
 
-        return 0;
+        return 1;
 }
 
 int config_parse_rlimit(