netfilter: nf_tables: reject unbound anonymous set before commit phase

Add a new list to track set transaction and to check for unbound
anonymous sets before entering the commit phase.

Bail out at the end of the transaction handling if an anonymous set
remains unbound.

Fixes: 96518518

 ("netfilter: add nftables")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>