IPsec interface MTU value

IPsec interfaces may calculate a different MTU value after upgrading from 6.2.

This change might cause an OSPF neighbor to not be established after upgrading. The workaround is to set mtu-ignore to enable on the OSPF interface's configuration:

config router ospf
    config ospf-interface
        edit "ipsce-vpnx"
            set mtu-ignore enable
        next
    end
end