extensions: libipt_DNAT/SNAT: fix "OOM" when do translation to nft

extensions: libipt_DNAT/SNAT: fix "OOM" when do translation to nft

When I want to translate SNAT target to nft rule, an error message
was printed out:
  # iptables-translate -A POSTROUTING -j SNAT --to-source 1.1.1.1
  iptables-translate v1.6.0: OOM

Because ipt_natinfo{} started with a xt_entry_target{}, so when we
get the ipt_natinfo pointer, we should use the target itself,
not its data pointer. Yes, it is a little tricky and it's different
with other targets.

Fixes: 7a0992da44cf ("src: introduce struct xt_xlate_{mt,tg}_params")
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>