[NETFILTER]: Trivial 2.6 tftp conntrack fix.

In 2.6, the TFTP conntrack helper returns -1 if the packet is too
short, but that is an invalid return code.  Return NF_ACCEPT
instead.

net/ipv4/netfilter/ip_conntrack_tftp.c

@@ -44,7 +44,7 @@ static int tftp_help(struct sk_buff *skb,
 
 	if (skb_copy_bits(skb, skb->nh.iph->ihl * 4 + sizeof(struct udphdr),
 			  &tftph, sizeof(tftph)) != 0)
-		return -1;
+		return NF_ACCEPT;
 
 	switch (ntohs(tftph.opcode)) {
 	/* RRQ and WRQ works the same way */