Detach the interface first, do vlan_unconfig() then.
Previously, another thread could get a pointer to the
interface by scanning the system-wide list and sleep
on the global vlan mutex held by vlan_unconfig().
The interface was gone by the time the other thread
woke up.

In order to be able to call vlan_unconfig() on a detached
interface, remove the purely cosmetic bzero'ing of IF_LLADDR
from the function because a detached interface has no addresses.

Noticed by: a stress-testing script by maxim
Reviewed by: glebius