netfilter: nf_conntrack: fix memory leak if sysctl registration fails

In nf_ct_l4proto_register_sysctl, if l4proto sysctl registration
fails, we have to make sure that we release the compat sysctl
table.

This can happen if TCP has been registered compat for IPv4, and
IPv6 compat registration fails.

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