netfilter: nf_tables: avoid false-positive lockdep splats with sets

Same as previous patch.  All set handling functions here can be called
with transaction mutex held (but not the rcu read lock).

The transaction mutex prevents concurrent add/delete, so this is fine.

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