dev: set iflink to 0 for virtual interfaces

Virtual interfaces are supposed to set an iflink value != of their ifindex.
It was not the case for some of them, like vxlan, bond or bridge.
Let's set iflink to 0 when dev->rtnl_link_ops is set.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>