netdev-linux: Do not log a warning if the device is down.

netdev-linux: Do not log a warning if the device is down.

In the userspace datapath we use tap devices as internal netdev.  The
datapath doesn't consider whether a device is up or down before sending
to it, and so far this hasn't been a problem.

Since Linux upstream commit 1bd4978a88ac("tun: honor IFF_UP in
tun_get_user()"), included in 4.4, writing to a tap device that is not
up sets errno to EIO.  This commit avoids printing a warning in this
case.

This fixes a failures in the system-userspace-testsuites.

Reported-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@ovn.org>