vxlan_core: make multicast helper take rip and ifindex explicitly

This patch changes multicast helpers to take rip and ifindex as input.
This is needed in future patches where rip can come from a pervni
structure while the ifindex can come from the vxlan device.

Signed-off-by: Roopa Prabhu <roopa@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>