[NETFILTER] ctnetlink: use size_t to make gcc-4.x happy

Make gcc-4.x happy. Use size_t instead of int. Thanks to Patrick McHardy
for the hint.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>