The original issue is reported at https://bugzilla.redhat.com/show_bug.cgi?id=2313696.

Description of problem:
In IPv6, it mandates that MTU is no less than 1280 octets.
However, RHOSP17.1 can create IPv6 subnet with the MTU less than 1280 octets.

We should deny such request for letting user know they have to configure the proper MTU to their network.

Version-Release number of selected component (if applicable):
OSP17.1

How reproducible:
Everytime.

Steps to Reproduce:
1. Deploy RHOSP17.1
2. Create a network with MTU:1200
3. Create a IPv6 subnet to the network.

Actual results:
The subnet can be created.

Expected results:
The subnet cannot be created.

Additional info:
The upstream fix is https://review.opendev.org/c/openstack/neutron/+/875809