netfilter: ipset: Fix warn: integer overflows 'sizeof(*map) + size * set->dsize'

Dan Carpenter reported that the static checker emits the warning

        net/netfilter/ipset/ip_set_list_set.c:600 init_list_set()
        warn: integer overflows 'sizeof(*map) + size * set->dsize'

Limit the maximal number of elements in list type of sets.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>