netfilter: nf_tables: use rhashtable_lookup() instead of rhashtable_lookup_fast()

Internally, rhashtable_lookup_fast() calls rcu_read_lock() then,
calls rhashtable_lookup(). so that in places where are guaranteed
by rcu read lock, rhashtable_lookup() is enough.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>