platform: change @plen field of NMPlatformIPxRoute to type guint8

On netlink layer, this field is uint8_t/uchar.

A larger (signed) plen makes no sense. Adjust the signatures
to have only guint8.

src/dhcp-manager/nm-dhcp-systemd.c

@@ -312,7 +312,8 @@ lease_to_ip4_config (const char *iface,
 				continue;
 			route.network = a.s_addr;
 
-			if (sd_dhcp_route_get_destination_prefix_length (routes[i], &plen) < 0)
+			if (   sd_dhcp_route_get_destination_prefix_length (routes[i], &plen) < 0
+			    || plen > 32)
 				continue;
 			route.plen = plen;
 

src/dhcp-manager/nm-dhcp-utils.c

@@ -60,7 +60,7 @@ ip4_process_dhcpcd_rfc3442_routes (const char *str,
 			*slash = '\0';
 			errno = 0;
 			rt_cidr = strtol (slash + 1, NULL, 10);
-			if ((errno == EINVAL) || (errno == ERANGE)) {
+			if (errno || rt_cidr > 32) {
 				nm_log_warn (LOGD_DHCP4, "DHCP provided invalid classless static route cidr: '%s'", slash + 1);
 				continue;
 			}

src/dhcp-manager/tests/test-dhcp-utils.c

@@ -208,6 +208,8 @@ ip4_test_route (NMIP4Config *ip4_config,
 	const NMPlatformIP4Route *route;
 	guint32 tmp;
 
+	g_assert (expected_prefix <= 32);
+
 	route = nm_ip4_config_get_route (ip4_config, route_num);
 	g_assert (inet_pton (AF_INET, expected_dest, &tmp) > 0);
 	g_assert (route->network == tmp);