netfilter: nf_ct_tcp: wrong multiplication of TCPOLEN_TSTAMP_ALIGNED in tcp_sack skips fastpath

The wrong multiplication of TCPOLEN_TSTAMP_ALIGNED by 4 skips the fast path
for the timestamp-only option. Bug reported by Michael M. Builov (netfilter
bugzilla #738).
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_proto_tcp.c

@@ -447,7 +447,7 @@ static void tcp_sack(const struct sk_buff *skb, unsigned int dataoff,
 	BUG_ON(ptr == NULL);
 
 	/* Fast path for timestamp-only option */
-	if (length == TCPOLEN_TSTAMP_ALIGNED*4
+	if (length == TCPOLEN_TSTAMP_ALIGNED
 	    && *(__be32 *)ptr == htonl((TCPOPT_NOP << 24)
 				       | (TCPOPT_NOP << 16)
 				       | (TCPOPT_TIMESTAMP << 8)