netdevsim: fix backwards compatibility in nsim_get_iflink()

The default ndo_get_iflink() implementation returns the current ifindex
of the netdev. But the overridden nsim_get_iflink() returns 0 if the
current nsim is not linked, breaking backwards compatibility for
userspace that depend on this behaviour.

Fix the problem by returning the current ifindex if not linked to a
peer.

Fixes: 8debcf58 ("netdevsim: add ndo_get_iflink() implementation")
Reported-by: Yu Watanabe <watanabe.yu@gmail.com>
Suggested-by: Yu Watanabe <watanabe.yu@gmail.com>
Signed-off-by: David Wei <dw@davidwei.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/netdevsim/netdev.c

@@ -324,7 +324,8 @@ static int nsim_get_iflink(const struct net_device *dev)
 
 	rcu_read_lock();
 	peer = rcu_dereference(nsim->peer);
-	iflink = peer ? READ_ONCE(peer->netdev->ifindex) : 0;
+	iflink = peer ? READ_ONCE(peer->netdev->ifindex) :
+			READ_ONCE(dev->ifindex);
 	rcu_read_unlock();
 
 	return iflink;