netfilter: nf_tables: use kfree_rcu(ptr, rcu) to release hooks in clean_net path

Use kfree_rcu(ptr, rcu) variant instead as described by ae089831
("netfilter: nf_tables: prefer kfree_rcu(ptr, rcu) variant").

Fixes: f9a43007 ("netfilter: nf_tables: double hook unregistration in netns path")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>