batman-adv: Enable LockLess TX for softif

The batadv interfaces are virtual interfaces which just tunnel the traffic
over other ethernet compatible interfaces. It doesn't need serialization
during the tx phase and is using RCU for most of its internal
datastructures. Since it doesn't have actual queues which could be locked
independently, the throughput gets significantly reduced by the extra lock
in the core net code.

8 parallel TCP connections forwarded by an IPQ4019 based hardware over
5GHz could reach:

* without LLTX: 349 Mibit/s
* with LLTX:    563 Mibit/s

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>