xia/u4id: cleanup and rename variable to match UDP
xia/u4id: use udp_set_csum

This patch complies with commits:
  - af5fcba7f38f3166392f4087ab734433c84f160b
    udp: Generic functions to set checksum
  - 28448b80456feafe07e2d05b6363b00f61f6171e
    net: Split sk_no_check into sk_no_check_{rx,tx}
xia/u4id: call udp_sock_create

This patch complies with commit:
  - 8024e02879ddd5042be02c70557f74cdc70b44b4
    udp: Add udp_sock_create for UDP tunnels to open listener socket