netfilter: nf_tables: missing iterator type in lookup walk

Add missing decorator type to lookup expression and tighten WARN_ON_ONCE
check in pipapo to spot earlier that this is unset.

Fixes: 29b359cf ("netfilter: nft_set_pipapo: walk over current view on netlink dump")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>