netfilter: nat: merge udp and udplite helpers

udplite nat was copied from udp nat, they are virtually 100% identical.
Not really surprising given udplite is just udp with partial csum coverage.

old:
   text    data     bss     dec     hex filename
  11606    1457     210   13273    33d9 nf_nat.ko
    330       0       2     332     14c nf_nat_proto_udp.o
    276       0       2     278     116 nf_nat_proto_udplite.o
new:
   text    data     bss     dec     hex filename
  11598    1457     210   13265    33d1 nf_nat.ko
    640       0       4     644     284 nf_nat_proto_udp.o

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>