netfilter: Fix memory leak in nf_register_net_hook

In the rare case that when it is a attempted to use a per network device
netfilter hook and the network device does not exist the newly allocated
structure can leak.

Be a good citizen and free the newly allocated structure in the error
handling code.

Fixes: 085db2c0 ("netfilter: Per network namespace netfilter hooks.")
Reported-by: kbuild@01.org
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/core.c

@@ -96,8 +96,10 @@ int nf_register_net_hook(struct net *net, const struct nf_hook_ops *reg)
 	new->priority = reg->priority;
 
 	nf_hook_list = find_nf_hook_list(net, reg);
-	if (!nf_hook_list)
+	if (!nf_hook_list) {
+		kfree(new);
 		return -ENOENT;
+	}
 
 	mutex_lock(&nf_hook_mutex);
 	list_for_each_entry(elem, nf_hook_list, list) {