netfilter: nf_nat: fix crash in nf_nat_csum

Fix crash in nf_nat_csum when mangling packets
in OUTPUT hook where skb->dev is not defined, it is set
later before POSTROUTING. Problem happens for CHECKSUM_NONE.
We can check device from rt but using CHECKSUM_PARTIAL
should be safe (skb_checksum_help).

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>