netfilter: nft_limit: fix packet ratelimiting

Credit calculations for the packet ratelimiting are not correct, as per
the applied ratelimit of 25/second and burst 8, a total of 33 packets
should have been accepted.  This is true in iptables(33) but not in
nftables (~65). For packet ratelimiting, use:

	div_u64(limit->nsecs, limit->rate) * limit->burst;

to calculate credit, just like in iptables' xt_limit does.

Moreover, use default burst in iptables, users are expecting similar
behaviour.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>