vxlan: fdb: allow specifying multiple destinations for zero MAC

The zero MAC entry in the fdb is used as default destination. With
multiple default destinations it is possible to use vxlan in
environments that disable multicast on the infrastructure level, e.g.
public clouds.

Signed-off-by: Mike Rapoport <mike.rapoport@ravellosystems.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>