netfilter: ipset: list:set: fix reference counter update

commit 02f815cb upstream.

The last element can be replaced or pushed off and in both
cases the reference counter must be updated.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>