netfilter: xt_nat: fix DNAT target for shifted portmap ranges

The commit 2eb0f624 ("netfilter: add NAT support for shifted
portmap ranges") did not set the checkentry/destroy callbacks for
the newly added DNAT target. As a result, rulesets using only
such nat targets are not effective, as the relevant conntrack hooks
are not enabled.
The above affect also nft_compat rulesets.
Fix the issue adding the missing initializers.

Fixes: 2eb0f624 ("netfilter: add NAT support for shifted portmap ranges")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>