netfilter: conntrack: add nf_ct_iter_data object for nf_ct_iterate_cleanup*()

This patch adds a structure to collect all the context data that is
passed to the cleanup iterator.

 struct nf_ct_iter_data {
       struct net *net;
       void *data;
       u32 portid;
       int report;
 };

There is a netns field that allows to clean up conntrack entries
specifically owned by the specified netns.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>