Fix IPv6 sets nftables translation

Fix IPv6 sets nftables translation

The parser assumes the set is an IPv4 ipset because IPSET_OPT_FAMILY is
not set.

 # ipset-translate restore < ./ipset-mwan3_set_connected_ipv6.dump
 add table inet global
 add set inet global mwan3_connected_v6 { type ipv6_addr; flags interval; }
 flush set inet global mwan3_connected_v6
 ipset v7.15: Error in line 4: Syntax error: '64' is out of range 0-32

Remove ipset_xlate_type_get(), call ipset_xlate_set_get() instead to
obtain the set type and family.

Reported-by: Florian Eckert <fe@dev.tdt.de>
Fixes: 325af556cd3a ("add ipset to nftables translation infrastructure")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>