Changing the MTU on a lagg device

[Ryan Stone]

I have a similar patch internally.  As best that I can tell, the
author of the code didn't want to go to the effort of implementing the
error cases on the MTU change, and thought that it was better to
explicitly error out if somebody tried to change the MTU rather than
causing random unexpected behaviour.  You can put me (rstone@) as a
reviewer in phabricator if you need your patch reviewed.