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.