netfilter: nf_tables: fix wrong transaction ordering in set elements

The transaction needs to be placed at the end of the commit list,
otherwise event notifications are reordered and we may crash when
releasing object via call_rcu.

This problem was introduced in 60319eb1

 ("netfilter: nf_tables: use new
transaction infrastructure to handle elements").

Reported-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>