net: openvswitch: use u64 for meter bucket

[ Upstream commit e57358873bb5d6caa882b9684f59140912b37dde ]

When setting the meter rate to 4+Gbps, there is an
overflow, the meters don't work as expected.

Cc: Pravin B Shelar <pshelar@ovn.org>
Cc: Andy Zhou <azhou@ovn.org>
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>