ipv6: Use rcu_barrier() on module unload.

The ipv6 module uses rcu_call() thus it should use rcu_barrier() on
module unload.
Acked-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NJesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/af_inet6.c

@@ -1284,6 +1284,8 @@ static void __exit inet6_exit(void)
 	proto_unregister(&udplitev6_prot);
 	proto_unregister(&udpv6_prot);
 	proto_unregister(&tcpv6_prot);
+
+	rcu_barrier(); /* Wait for completion of call_rcu()'s */
 }
 module_exit(inet6_exit);