netfilter: ipset: Resolve missing-field-initializer warnings

Resolve missing-field-initializer warnings by providing a
directed initializer.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>