net: remove iflink field from struct net_device

Now that all users of iflink have the ndo_get_iflink handler available, it's
possible to remove this field.

By default, dev_get_iflink() returns the ifindex of the interface.

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