netfilter: xt_nat: fix incorrect hooks for SNAT and DNAT targets

In (c7232c99 netfilter: add protocol independent NAT core), the
hooks were accidentally modified:

SNAT hooks are POST_ROUTING and LOCAL_IN (before it was LOCAL_OUT).
DNAT hooks are PRE_ROUTING and LOCAL_OUT (before it was LOCAL_IN).

Signed-off-by: Elison Niven <elison.niven@cyberoam.com>
Signed-off-by: Sanket Shah <sanket.shah@cyberoam.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>