openvswitch: Use RCU callback when detaching netdevices.

Currently, each time a device is detached from an OVS datapath
we call synchronize RCU before freeing associated data structures.
However, if a bridge is deleted (which detaches all ports) when
many devices are connected then there can be a long delay.  This
switches to use call_rcu() to group the cost together.

Reported-by: Justin Pettit <jpettit@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
diff --git a/net/openvswitch/vport-netdev.c b/net/openvswitch/vport-netdev.c
index a903348..a9327e2e 100644
--- a/net/openvswitch/vport-netdev.c
+++ b/net/openvswitch/vport-netdev.c
@@ -114,6 +114,15 @@
 	return ERR_PTR(err);
 }
 
+static void free_port_rcu(struct rcu_head *rcu)
+{
+	struct netdev_vport *netdev_vport = container_of(rcu,
+					struct netdev_vport, rcu);
+
+	dev_put(netdev_vport->dev);
+	ovs_vport_free(vport_from_priv(netdev_vport));
+}
+
 static void netdev_destroy(struct vport *vport)
 {
 	struct netdev_vport *netdev_vport = netdev_vport_priv(vport);
@@ -122,10 +131,7 @@
 	netdev_rx_handler_unregister(netdev_vport->dev);
 	dev_set_promiscuity(netdev_vport->dev, -1);
 
-	synchronize_rcu();
-
-	dev_put(netdev_vport->dev);
-	ovs_vport_free(vport);
+	call_rcu(&netdev_vport->rcu, free_port_rcu);
 }
 
 const char *ovs_netdev_get_name(const struct vport *vport)