netfilter: nf_tables: prefer kfree_rcu(ptr, rcu) variant

While kfree_rcu(ptr) _is_ supported, it has some limitations.

Given that 99.99% of kfree_rcu() users [1] use the legacy
two parameters variant, and @catchall objects do have an rcu head,
simply use it.

Choice of kfree_rcu(ptr) variant was probably not intentional.

[1] including calls from net/netfilter/nf_tables_api.c

Fixes: aaa31047 ("netfilter: nftables: add catch-all set element support")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>