netfilter: flowtable: prefer refcount_inc

With refcount_inc_not_zero, we'd also need a smp_rmb or similar,
followed by a test of the CONFIRMED bit.

However, the ct pointer is taken from skb->_nfct, its refcount must
not be 0 (else, we'd already have a use-after-free bug).

Use refcount_inc() instead to clarify the ct refcount is expected to
be at least 1.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_flow_table_core.c

@@ -53,14 +53,14 @@ struct flow_offload *flow_offload_alloc(struct nf_conn *ct)
 {
 	struct flow_offload *flow;
 
-	if (unlikely(nf_ct_is_dying(ct) ||
-	    !refcount_inc_not_zero(&ct->ct_general.use)))
+	if (unlikely(nf_ct_is_dying(ct)))
 		return NULL;
 
 	flow = kzalloc(sizeof(*flow), GFP_ATOMIC);
 	if (!flow)
-		goto err_ct_refcnt;
+		return NULL;
 
+	refcount_inc(&ct->ct_general.use);
 	flow->ct = ct;
 
 	flow_offload_fill_dir(flow, FLOW_OFFLOAD_DIR_ORIGINAL);
@@ -72,11 +72,6 @@ struct flow_offload *flow_offload_alloc(struct nf_conn *ct)
 		__set_bit(NF_FLOW_DNAT, &flow->flags);
 
 	return flow;
-
-err_ct_refcnt:
-	nf_ct_put(ct);
-
-	return NULL;
 }
 EXPORT_SYMBOL_GPL(flow_offload_alloc);