If the configuration option "ovn.ovn_emit_need_to_frag" is set, write
always the "LSP.options.gateway_mtu" value, using the minimum MTU
value of all networks connected to the router. That will limit the
N/S traffic MTU regardless if the gateway network MTU is greater or
less than the internal network MTU.
Closes-Bug: #2032817
Change-Id: Ia1f0c0955d2fd90c7ff6307b1bc9819721ee2f9b