[DM]: Fix for recent regression issue, ip_alloc issue

This is regression issue introduced after changing the prototype of ip_alloc API.

Change-Id: I2cc7a9a42157a25c2649e56be6906189be8a9f19
Closes-Bug: #1646344

src/config/device-manager/device_manager/db.py

@@ -293,6 +293,7 @@ def evaluate_vn_irb_ip_map(self, vn_set, fwd_mode, ip_used_for, ignore_external=
 
         for vn_subnet in create_set:
             (vn_uuid, subnet_prefix) = vn_subnet.split(':', 1)
+            vn = VirtualNetworkDM.get(vn_uuid)
             subnet_uuid = vn.gateways[subnet_prefix].get('subnet_uuid')
             (sub, length) = subnet_prefix.split('/')
             ip_addr = self.reserve_ip(vn_uuid, subnet_uuid)