Should vlan_input() ever be called with ifp pointing to a non-Ethernet
interface, do not just assign -1 to tag because it breaks the logic of
the code to follow. The better way is to handle this case as an unsupported
protocol and return unless INVARIANTS is in effect and we can panic.
Panic is good there because the scenario can happen only because of a
coding error elsewhere.

We also should show the interface name in the panic message for easier
debugging of the problem, should it ever emerge.

Submitted by: qingli (initially)