netfilter: ipset: Use kmalloc() in comment extension helper

Allocate memory with kmalloc() rather than kzalloc(): the string
is immediately initialized so it is unnecessary to zero out
the allocated memory area.

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

Suggested-by: Sergey Popovich <popovich_sergei@mail.ua>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>