net/xia: drop calls to skb_net()

When CONFIG_NET_NS is enabled, skb_net() changes its behavior depending
on CONFIG_IP_VS_DEBUG.
This leads to bugs that depend on how the kernel is compiled.

This patch replaces skb_net() to the most appropriate method to obtain
a pointer to struct net at each needed occasion.

In special, xip_route() and xip_route_with_a_redirect() now requires
struct net *net as a parameter.
XIA/U4ID: change checksum_disabled to no_check 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