vxlan: fix a regression of igmp join

This is a regression introduced by:

	commit 3fc2de2f
	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>