netfilter: xt_rateest: Better struct xt_rateest layout

We currently dirty two cache lines in struct xt_rateest, this hurts SMP
performance.

This patch moves lock/bstats/rstats at beginning of structure so that
they share a single cache line.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>