netfilter: nf_ct_generic: add generic_kmemdup_sysctl_table function

This patch is a cleanup. It adds generic_kmemdup_sysctl_table to
split code into smaller chunks. Yet it prepares introduction
of nf_conntrack_proto_*_sysctl.c.

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