netfilter: ipset: restore allowing 64 clashing elements in hash:net,iface

The commit 510841da ("netfilter: ipset: enforce documented limit to
prevent allocating huge memory") was too strict and prevented to add up to
64 clashing elements to a hash:net,iface type of set. This patch fixes the
issue and now the type behaves as documented.

Fixes: 510841da ("netfilter: ipset: enforce documented limit to prevent allocating huge memory")
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>