vxlan: fix udp checksum control

So far, passing "rxcsum" and "txcsum" had no effect.

Fixes: 95ab18e012 ("vxlan: add options to enable and disable UDP
checksums")

Signed-off-by: Johannes Kimmel <fff@bareminimum.eu>
[add Fixes:]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 0453c3866feb701160bbab4ecf9762c5a3038503)

package/network/config/vxlan/files/vxlan.sh

@@ -129,6 +129,8 @@ vxlan_generic_init_config() {
 	proto_config_add_int "ttl"
 	proto_config_add_int "tos"
 	proto_config_add_int "mtu"
+	proto_config_add_boolean "rxcsum"
+	proto_config_add_boolean "txcsum"
 	proto_config_add_string "macaddr"
 }