Insert a workaround for per-ifp ioctl being called w/o NET_LOCK().

Insert a workaround for per-ifp ioctl being called w/o NET_LOCK().

An example of such code path is vlan_destroy() where we don't want to
grab the lock because this driver is already NET_LOCK()-free.

The real solution to this problem is to stop holding the NET_LOCK() around
driver *_ioctl() routines.

Problem reported & fix tested by Élie Bouttier, ok visa@, bluhm@