[NETFILTER] nfnetlink: Use kzalloc

These is a cleanup patch, kzalloc can be used in a couple of cases

Signed-off-by: Samir Bellabes <sbellabes@mandriva.com>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>