batman,rcu: convert call_rcu(gw_node_free_rcu) to kfree_rcu

The RCU callback gw_node_free_rcu() just calls kfree(), so we can use
kfree_rcu() instead of call_rcu().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Marek Lindner <lindner_marek@yahoo.de>
Cc: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Sven Eckelmann <sven@narfation.org>