netfilter: nf_conntrack: fix count leak in error path of __nf_conntrack_alloc

We have to decrement the conntrack counter if we fail to access the
zone extension.
Reported-by: NTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/nf_conntrack_core.c

@@ -735,6 +735,7 @@ __nf_conntrack_alloc(struct net *net, u16 zone,
 
 #ifdef CONFIG_NF_CONNTRACK_ZONES
 out_free:
+	atomic_dec(&net->ct.count);
 	kmem_cache_free(net->ct.nf_conntrack_cachep, ct);
 	return ERR_PTR(-ENOMEM);
 #endif