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>

net/openvswitch/vport-netdev.h

@@ -20,12 +20,15 @@
 #define VPORT_NETDEV_H 1
 
 #include <linux/netdevice.h>
+#include <linux/rcupdate.h>
 
 #include "vport.h"
 
 struct vport *ovs_netdev_get_vport(struct net_device *dev);
 
 struct netdev_vport {
+	struct rcu_head rcu;
+
 	struct net_device *dev;
 };