vxlan: fix a regression of igmp join

This is a regression introduced by:

	commit 3fc2de2faba387218bdf9dbc6b13f513ac3b060a
	Author: stephen hemminger <stephen@networkplumber.org>
	Date:   Thu Jul 18 08:40:15 2013 -0700

	    vxlan: fix igmp races

Before this commit, the old code was:

       if (vxlan_group_used(vn, vxlan->default_dst.remote_ip))
               ip_mc_join_group(sk, &mreq);
       else
               ip_mc_leave_group(sk, &mreq);

therefore we shoud check vxlan_group_used(), not its opposite,
for igmp_join.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -1386,7 +1386,7 @@ static int vxlan_open(struct net_device *dev)
 		return -ENOTCONN;
 
 	if (IN_MULTICAST(ntohl(vxlan->default_dst.remote_ip)) &&
-	    ! vxlan_group_used(vn, vxlan->default_dst.remote_ip)) {
+	    vxlan_group_used(vn, vxlan->default_dst.remote_ip)) {
 		vxlan_sock_hold(vs);
 		dev_hold(dev);
 		queue_work(vxlan_wq, &vxlan->igmp_join);