netfilter: nf_ct_icmpv6: add icmpv6_kmemdup_sysctl_table function

Split sysctl function into smaller chucks to cleanup code and prepare
patches to reduce ifdef pollution.

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