ipv6: Use rcu_barrier() on module unload.

The ipv6 module uses rcu_call() thus it should use rcu_barrier() on
module unload.

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>