vxlan: source compatiablity with IFLA_VXLAN_GROUP (v2)

Source compatiability for build iproute2 was broken by:
  commit c7995c43
  Author: Atzm Watanabe <atzm@stratosphere.co.jp>
    vxlan: Allow setting destination to unicast address.

Since this commit has not made it upstream (still net-next),
and better to avoid gratitious changes to exported API's;
go back to original definition, and add a comment.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>