netfilter: ipset: Fixing unnamed union init

In continue to proposed Vinson Lee's post [1], this patch fixes compilation
issues founded at gcc 4.4.7. The initialization of .cidr field of unnamed
unions causes compilation error in gcc 4.4.x.

References

Visible links
[1] https://lkml.org/lkml/2015/7/5/74



Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>