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>