[DHCP agent] Fix route to OVN metadata port for non-isolated networks

This was missed in the original fix[1] during 5th patch set.
When ovn metadata port exists it's ip should be used as route
irrespective of subnet is isolated or not.

[1] https://review.opendev.org/c/openstack/neutron/+/886988

Related-Bug: #1982569
Related-Bug: https://bugzilla.redhat.com/show_bug.cgi?id=2213862
Change-Id: Icd84685c37fffa20e4fc9c5522f77bc63e2565f2
(cherry picked from commit 56172ed)

Author: karelyatin

neutron/agent/linux/dhcp.py

@@ -1289,7 +1289,7 @@ def _generate_opts_per_subnet(self):
                     if subnet_dhcp_ip:
                         metadata_route_ip = subnet_dhcp_ip
 
-                if not isolated_subnets[subnet.id] and gateway:
+                elif not isolated_subnets[subnet.id] and gateway:
                     metadata_route_ip = gateway
 
                 if metadata_route_ip: