netfilter: ipset: Make sure element data size is a multiple of u32

Data for hashing required to be array of u32. Make sure that
element data always multiple of u32.

Ported from a patch proposed by Sergey Popovich <popovich_sergei@mail.ua>.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>