vxlan: allow live mac address change

The VXLAN pseudo-device doesn't care if the mac address changes
when device is up.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>