vxlan: fix a soft lockup in vxlan module removal

This is a regression introduced by:

	commit fe5c3561
	Author: stephen hemminger <stephen@networkplumber.org>
	Date:   Sat Jul 13 10:18:18 2013 -0700

	    vxlan: add necessary locking on device removal

The problem is that vxlan_dellink(), which is called with RTNL lock
held, tries to flush the workqueue synchronously, but apparently
igmp_join and igmp_leave work need to hold RTNL lock too, therefore we
have a soft lockup!

As suggested by Stephen, probably the flush_workqueue can just be
removed and let the normal refcounting work. The workqueue has a
reference to device and socket, therefore the cleanups should work
correctly.
Suggested-by: Stephen Hemminger <stephen@networkplumber.org>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: David S. Miller <davem@davemloft.net>
Tested-by: Cong Wang <amwang@redhat.com>
Signed-off-by: Cong Wang <amwang@redhat.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -1793,8 +1793,6 @@ static void vxlan_dellink(struct net_device *dev, struct list_head *head)
 	struct vxlan_net *vn = net_generic(dev_net(dev), vxlan_net_id);
 	struct vxlan_dev *vxlan = netdev_priv(dev);
 
-	flush_workqueue(vxlan_wq);
-
 	spin_lock(&vn->sock_lock);
 	hlist_del_rcu(&vxlan->hlist);
 	spin_unlock(&vn->sock_lock);