veth: Fix carrier detect

The current implementation of carrier detect in veth is broken.
It reports the link is down until both sides of the veth pair
are administatively up and then forever after it reports link up.

So fix veth so that it only reports link up when both interfaces
of the pair are administratively up.

Signed-off-by: Eric Biederman <ebiederm@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>