netfilter: nf_conntrack: add nf_ct_kfree_compat_sysctl_table

This patch is a cleanup.

It adds nf_ct_kfree_compat_sysctl_table to release l4proto's
compat sysctl table and set the compat sysctl table point to NULL.

This new function will be used by follow-up patches.

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