netfilter: nat: fix udp checksum corruption

Due to copy&paste error nf_nat_mangle_udp_packet passes IPPROTO_TCP,
resulting in incorrect udp checksum when payload had to be mangled.

Fixes: dac3fe72 ("netfilter: nat: remove csum_recalc hook")
Reported-by: NMarc Haber <mh+netdev@zugschlus.de>
Tested-by: NMarc Haber <mh+netdev@zugschlus.de>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_nat_helper.c

@@ -170,7 +170,7 @@ nf_nat_mangle_udp_packet(struct sk_buff *skb,
 	if (!udph->check && skb->ip_summed != CHECKSUM_PARTIAL)
 		return true;
 
-	nf_nat_csum_recalc(skb, nf_ct_l3num(ct), IPPROTO_TCP,
+	nf_nat_csum_recalc(skb, nf_ct_l3num(ct), IPPROTO_UDP,
 			   udph, &udph->check, datalen, oldlen);
 
 	return true;