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>