netfilter: ipset: take into account cidr value for the from address when creating the set

When creating a set from a range expressed as a network like
10.1.1.172/29, the from address was taken as the IP address part and
not masked with the netmask from the cidr.

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