How about this patch (still compiling, so untested)?

Its based on Rick's suggestion of using an operational and a desired
MTU. The operational MTU is simply the net_device's MTU and is always
set to the minimum of the lower device's MTU and the desired MTU.
The desired MTU can be changed by configuring a MTU for the VLAN device
manually.

The remaining question is whether we should report the desired MTU to
userspace using the netlink interface, or maybe print a message when
the MTU is limited by the lower device.