netfilter: nf_conntrack: fix nf_conntrack_l3proto_register

Before commit 2c352f44
(netfilter: nf_conntrack: prepare namespace support for
l4 protocol trackers), we register sysctl before register
protocol tracker. Thus, if sysctl is registration fails,
the protocol tracker will not be registered.

After that commit, if sysctl registration fails, protocol
registration still remains, so we leave things in intermediate
state.

To fix this, this patch registers sysctl before protocols.
And if protocol registration fail, sysctl is unregistered.

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