Use IFCAP_VLAN_MTU and IFCAP_VLAN_HWTAGGING capabilities:
Use IFCAP_VLAN_MTU and IFCAP_VLAN_HWTAGGING capabilities:

LINK0 disappears; we now set IFCAP_VLAN_HWTAGGING at ifnet->if_capabilities
in the Ethernet driver for cards/drivers which support hardware tagging.

MTU ambiguity disppears; we now set IFCAP_VLAN_MTU in the Ethernet driver
when we know the chip will not truncate/discard vlan-sized frames.

Only allow the MTU to be changed within the scope of the parent interface's
MTU.  (Here we also take into account IFCAP_VLAN_MTU)

Propagate hardware-assisted IP/TCP/UDP checksumming flags to the vlan interface
if the card supports hardware tagging (from NetBSD)

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars