Use a callback to set up a vlan interface so that "vlan" and "vlandev" commands can be specified in any order. This makes the code more compact and clear as well.
Use a callback to set up a vlan interface so that "vlan"
and "vlandev" commands can be specified in any order.
This makes the code more compact and clear as well.

Improve error check on vlan argument.

MFC after:	2 weeks

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