netfilter: nft_flow_offload: add entry to flowtable after confirmation

This is fixing flow offload for UDP traffic where packets only follow
one single direction.

The flow_offload_fixup_tcp() mechanism works fine in case that the
offloaded entry remains in SYN_RECV state, given sequence tracking is
reset and that conntrack handles syn+ack packets as a retransmission, ie.

	sES + synack => sIG

for reply traffic.

Fixes: a3c90f7a ("netfilter: nf_tables: flow offload expression")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_flow_offload.c

@@ -94,8 +94,7 @@ static void nft_flow_offload_eval(const struct nft_expr *expr,
 	if (help)
 		goto out;
 
-	if (ctinfo == IP_CT_NEW ||
-	    ctinfo == IP_CT_RELATED)
+	if (!nf_ct_is_confirmed(ct))
 		goto out;
 
 	if (test_and_set_bit(IPS_OFFLOAD_BIT, &ct->status))