netfilter: ipset: off by one in ip_set_nfnl_get_byindex()

The ->ip_set_list[] array is initialized in ip_set_net_init() and it
has ->ip_set_max elements so this check should be >= instead of >
otherwise we are off by one.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>