garp: remove one synchronize_rcu() call

Speedup vlan dismantling in CONFIG_VLAN_8021Q_GVRP=y cases,
by using a call_rcu() to free the memory instead of waiting with
expensive synchronize_rcu() [ while RTNL is held ]

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Ben Greear <greearb@candelatech.com>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/garp.h

@@ -108,6 +108,7 @@ struct garp_applicant {
 
 struct garp_port {
 	struct garp_applicant __rcu	*applicants[GARP_APPLICATION_MAX + 1];
+	struct rcu_head			rcu;
 };
 
 extern int	garp_register_application(struct garp_application *app);

net/802/garp.c

@@ -544,6 +544,11 @@ static int garp_init_port(struct net_device *dev)
 	return 0;
 }
 
+static void garp_kfree_rcu(struct rcu_head *head)
+{
+	kfree(container_of(head, struct garp_port, rcu));
+}
+
 static void garp_release_port(struct net_device *dev)
 {
 	struct garp_port *port = rtnl_dereference(dev->garp_port);
@@ -554,8 +559,7 @@ static void garp_release_port(struct net_device *dev)
 			return;
 	}
 	rcu_assign_pointer(dev->garp_port, NULL);
-	synchronize_rcu();
-	kfree(port);
+	call_rcu(&port->rcu, garp_kfree_rcu);
 }
 
 int garp_init_applicant(struct net_device *dev, struct garp_application *appl)