[NET]: Remove double dev->flags checking when calling dev_close()

The unregister_netdevice() and dev_change_net_namespace()
both check for dev->flags to be IFF_UP before calling the
dev_close(), but the dev_close() checks for IFF_UP itself,
so remove those unneeded checks.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>