batman-adv: Add missing kerneldoc for extack

The parameter extack was added to batadv_softif_slave_add without adding
the kernel-doc for it. This caused kernel-doc warnings.
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/soft-interface.c

@@ -863,6 +863,7 @@ static int batadv_softif_init_late(struct net_device *dev)
  * batadv_softif_slave_add - Add a slave interface to a batadv_soft_interface
  * @dev: batadv_soft_interface used as master interface
  * @slave_dev: net_device which should become the slave interface
+ * @extack: extended ACK report struct
  *
  * Return: 0 if successful or error otherwise.
  */