netfilter: nf_nat: remove obsolete check in nf_nat_mangle_udp_packet()

The packet size check originates from a time when UDP helpers could
accidentally mangle incorrect packets (NEWNAT) and is unnecessary
nowadays since the conntrack helpers invoke the NAT helpers for the
proper packet directly.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nf_nat_helper.c

@@ -253,12 +253,6 @@ nf_nat_mangle_udp_packet(struct sk_buff *skb,
 	struct udphdr *udph;
 	int datalen, oldlen;
 
-	/* UDP helpers might accidentally mangle the wrong packet */
-	iph = ip_hdr(skb);
-	if (skb->len < iph->ihl*4 + sizeof(*udph) +
-			       match_offset + match_len)
-		return 0;
-
 	if (!skb_make_writable(skb, skb->len))
 		return 0;