netfilter: nf_tables: avoid false-positive lockdep splats in set walker

Its not possible to add or delete elements from hash and bitmap sets,
as long as caller is holding the transaction mutex, so its ok to iterate
the list outside of rcu read side critical section.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>