We check that all the member interfaces have the same MTU on attach to the bridge but the interface can still be changed afterwards.
We check that all the member interfaces have the same MTU on attach to the
bridge but the interface can still be changed afterwards.

This falls under the 'dont do that' category but log an warning when INVARIANTS
is defined.

Approved by:	mlaier (mentor)
MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase