netfilter: WARN about wrong usage of sequence number adjustments

Since commit 41d73ec0 (netfilter: nf_conntrack: make sequence
number adjustments usuable without NAT), the sequence number extension
is dynamically allocated.

Instead of dying, give a WARN splash, in case of wrong usage of the
seqadj code, e.g. when forgetting to allocate via nfct_seqadj_ext_add().

Wrong usage have been seen in the IPVS code path.
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>

net/netfilter/nf_conntrack_seqadj.c

@@ -36,6 +36,11 @@ int nf_ct_seqadj_set(struct nf_conn *ct, enum ip_conntrack_info ctinfo,
 	if (off == 0)
 		return 0;
 
+	if (unlikely(!seqadj)) {
+		WARN(1, "Wrong seqadj usage, missing nfct_seqadj_ext_add()\n");
+		return 0;
+	}
+
 	set_bit(IPS_SEQ_ADJUST_BIT, &ct->status);
 
 	spin_lock_bh(&ct->lock);