netfilter: nft_dynset: disallow object maps

Do not allow to insert elements from datapath to objects maps.

Fixes: 8aeff920

 ("netfilter: nf_tables: add stateful object reference to set elements")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>