netdevsim: add ndo_get_iflink() implementation

Add an implementation for ndo_get_iflink() in netdevsim that shows the
ifindex of the linked peer, if any.

Signed-off-by: David Wei <dw@davidwei.uk>
Reviewed-by: Maciek Machnikowski <maciek@machnikowski.net>
Signed-off-by: David S. Miller <davem@davemloft.net>