net/vxlan: Use the underlying device index when joining/leaving multicast groups

The socket calls from vxlan to join/leave multicast group aren't
using the index of the underlying device, as a result the stack uses
the first interface that is up. This results in vxlan being non functional
over a device which isn't the 1st to be up.
Fix this by providing the iflink field to the vxlan instance
to the multicast calls.
Signed-off-by: Yan Burman <yanb@mellanox.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -506,6 +506,7 @@ static int vxlan_join_group(struct net_device *dev)
 	struct sock *sk = vn->sock->sk;
 	struct ip_mreqn mreq = {
 		.imr_multiaddr.s_addr	= vxlan->gaddr,
+		.imr_ifindex		= vxlan->link,
 	};
 	int err;
 
@@ -533,6 +534,7 @@ static int vxlan_leave_group(struct net_device *dev)
 	struct sock *sk = vn->sock->sk;
 	struct ip_mreqn mreq = {
 		.imr_multiaddr.s_addr	= vxlan->gaddr,
+		.imr_ifindex		= vxlan->link,
 	};
 
 	/* Only leave group when last vxlan is done. */