openvswitch: meter: fix the incorrect calculation of max delta_t

Max delat_t should be the full_bucket/rate instead of the full_bucket.
Also report EINVAL if the rate is zero.

Fixes: 96fbc13d

 ("openvswitch: Add meter infrastructure")
Cc: Andy Zhou <azhou@ovn.org>
Signed-off-by: zhangliping <zhangliping02@baidu.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>