openvswitch: Use IS_ERR instead of IS_ERR_OR_NULL

Fix smatch warning:

net/openvswitch/meter.c:427 ovs_meter_cmd_set() warn: passing zero to 'PTR_ERR'

dp_meter_create() never returns NULL, use IS_ERR
instead of IS_ERR_OR_NULL to fix this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Link: https://lore.kernel.org/r/20201031060153.39912-1-yuehaibing@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>