net: openvswitch: suitable access to the dp_meters

To fix the following sparse warning:
| net/openvswitch/meter.c:109:38: sparse: sparse: incorrect type
| in assignment (different address spaces) ...
| net/openvswitch/meter.c:720:45: sparse: sparse: incorrect type
| in argument 1 (different address spaces) ...

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>