netfilter: ipset: allow a 0 netmask with hash_netiface type

Jozsef says:
 The correct behaviour is that if we have
 ipset create test1 hash:net,iface
 ipset add test1 0.0.0.0/0,eth0
 iptables -A INPUT -m set --match-set test1 src,src

 then the rule should match for any traffic coming in through eth0.

This removes the -EINVAL runtime test to make matching work
in case packet arrived via the specified interface.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1297092


Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>