netfilter: nf_ct_udplite: add udplite_kmemdup_sysctl_table function

This cleans up nf_conntrack_l4proto_udplite[4,6] and it prepares
the moving of the sysctl code to nf_conntrack_proto_*_sysctl.c
to reduce the ifdef pollution.

And use nf_proto_net.users to identify if it's the first time
we use the nf_proto_net, in that case, we initialize it.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>