netfilter: ctnetlink: fix error return code in init path

Initialize return variable before exiting on an error path.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
(
if@p1 (\(ret < 0\|ret != 0\))
 { ... return ret; }
|
ret@p1 = 0
)
... when != ret = e1
    when != &ret
*if(...)
{
  ... when != ret = e2
      when forall
 return ret;
}

// </smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_netlink.c

@@ -2790,7 +2790,8 @@ static int __init ctnetlink_init(void)
 		goto err_unreg_subsys;
 	}
 
-	if (register_pernet_subsys(&ctnetlink_net_ops)) {
+	ret = register_pernet_subsys(&ctnetlink_net_ops);
+	if (ret < 0) {
 		pr_err("ctnetlink_init: cannot register pernet operations\n");
 		goto err_unreg_exp_subsys;
 	}