netfilter: ipset: Fix parallel resizing and listing of the same set

When elements added to a hash:* type of set and resizing triggered,
parallel listing could start to list the original set (before resizing)
and "continue" with listing the new set. Fix it by references and
using the original hash table for listing. Therefore the destroying of
the original hash table may happen from the resizing or listing functions.

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