netfilter: ipset: fix adding ranges to hash types

When ranges are added to hash types, the elements may trigger rehashing
the set. However, the last successfully added element was not kept track
so the adding started again with the first element after the rehashing.

Bug reported by Mr Dash Four.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>