netfilter: ipset: Support updating extensions when the set is full

When the set was full (hash type and maxelem reached), it was not
possible to update the extension part of already existing elements.
The patch removes this limitation.

Fixes: https://bugzilla.netfilter.org/show_bug.cgi?id=880
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>