Partially revert the previous modification on vlan mtu. The function job is to find the biggest mtu between vlans and let it do that
Properly respect other configured MTUs for other vlans. Properly respect parent of vlan MTU if configured. Also avoid errors when possible. This helps VLANs MTU handling but all the other interfaces as gre/gif/... needs the same handling. It is better to require reboot on MTU changes especially on complex configurations.
Actually consider parentmtu 0 here to get the real value when unassgined
Actually get the correct value here!
Move these functions nearby since thy are related