Use the added dev_alloc_name() call to create tunnel device name,
rather than iterate in a hand-made loop with an artificial limit.

Thanks Patrick for noticing this.

[ The way this works is, when the device is actually registered,
  the generic code noticed the '%' in the name and invokes
  dev_alloc_name() to fully resolve the name.  -DaveM ]
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>