optimize: infer family for nat mapping

optimize: infer family for nat mapping

Infer family from key in nat mapping, otherwise nat mapping via merge
breaks since family is not specified.

Merging:
fw-test-bug2.nft:4:9-78:         iifname enp2s0 ip daddr 72.2.3.66 tcp dport 53122 dnat to 10.1.1.10:22
fw-test-bug2.nft:5:9-77:         iifname enp2s0 ip daddr 72.2.3.66 tcp dport 443 dnat to 10.1.1.52:443
fw-test-bug2.nft:6:9-75:         iifname enp2s0 ip daddr 72.2.3.70 tcp dport 80 dnat to 10.1.1.52:80
into:
        dnat ip to iifname . ip daddr . tcp dport map { enp2s0 . 72.2.3.66 . 53122 : 10.1.1.10 . 22, enp2s0 . 72.2.3.66 . 443 : 10.1.1.52 . 443, enp2s0 . 72.2.3.70 . 80 : 10.1.1.52 . 80 }

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1657
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>