duplicately fetch subnet_id in get_subnet_for_dvr

[ZongKai LI]

In https://github.com/openstack/neutron/blob/77a6d114eae9de8078b358a9bd8502fb7c393641/neutron/db/dvr_mac_db.py#L159-L163, get_subnet_for_dvr will try to get subnet_id when fixed_ips is not None:
    ...
    def get_subnet_for_dvr(self, context, subnet, fixed_ips=None):
        if fixed_ips:
            subnet_data = fixed_ips[0]['subnet_id']
        else:
            subnet_data = subnet
    ...

But checking its callers :
https://github.com/openstack/neutron/blob/77a6d114eae9de8078b358a9bd8502fb7c393641/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_dvr_neutron_agent.py#L509-L531 ,
and
https://github.com/openstack/neutron/blob/77a6d114eae9de8078b358a9bd8502fb7c393641/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_dvr_neutron_agent.py#L366-L380 , they all have similar logic:
        ...
        fixed_ip = fixed_ips[0]
        ...
        subnet_uuid = fixed_ip['subnet_id']
        ...
            subnet_info = self.plugin_rpc.get_subnet_for_dvr(
                self.context, subnet_uuid, fixed_ips=fixed_ips)

subnet_id has already be fetched and passed into get_subnet_for_dvr. So in get_subnet_for_dvr, there is no need to fetch again.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/263563

[OpenStack Infra (hudson-openstack)]

Change abandoned by Armando Migliaccio (<email address hidden>) on branch: master
Review: https://review.openstack.org/263563
Reason: This review is > 4 weeks without comment, and failed Jenkins the last time it was checked. We are abandoning this for now. Feel free to reactivate the review by pressing the restore button and leaving a 'recheck' comment to get fresh test results.

[Launchpad Janitor (janitor)]

[Expired for neutron because there has been no activity for 60 days.]