netfilter: nft_numgen: start round robin from zero

Currently we start round robin from 1, but it's better to start round
robin from 0. This is to keep consistent with xt_statistic in iptables.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>