datapath: meter: fix the incorrect calculation of max delta_t

datapath: meter: fix the incorrect calculation of max delta_t

Upstream commit:
    commit ddc502dfed600bff0b61d899f70d95b76223fdfc
    Author: zhangliping <zhangliping02@baidu.com>
    Date:   Fri Mar 9 10:08:50 2018 +0800

    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: 96fbc13d7e77 ("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>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>