LAGG MTU not set correctly when it has child QinQ interfaces
Issue number 2786 (https://redmine.pfsense.org/issues/2786) describes an issue where the MTU on the parent interfaces of a LAGG or VLAN were not being set correctly. This was resolved for the most part, but the solution didn't apply correctly for an instance where you have a QinQ on top of a LAGG. The MTU of the QinQ interfaces is not being read and hence the MTU of the parent interfaces and LAGG as a whole default to 1500.
I've managed to use the changes made in that bug to, for the most part, resolve this. Apologies, I'm not a programmer and don't have a clue how to create a diff or pull request. So for the moment will just upload the entire interface.inc that I have modified.
The changes I've made will check for any child QinQs on a LAGG, check their MTU values and set the LAGG MTU accordingly. Ideally, this would be modified slightly, so that if there are Child QinQs and no MTU has been explicitly set on them, the LAGG is set to 1504 to allow for the additional header overhead of the QinQ. I might take a look at doing this later.