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: Marc Haber <mh+netdev@zugschlus.de>
Tested-by: Marc Haber <mh+netdev@zugschlus.de>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>