Add IFNET_EVENT_UPDATE_BAUDRATE event and use it to update if_baudrate for vlan interfaces created on the LACP lagg.

[wma]

Summary

Putting the patch back to discussion. BugID: Bugzilla 193953

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=193953

@ae Do you think it's good enough to be merged, or you'd like to have it updated?

[ae]

I think the patch needs to be updated to reflect epoch related changes in the vlan code.
We use this patch. But I did not checked is it applicable to recent CURRENT or not.

[wma]

I've updated the patch with epoch_wait changes.