dhcp: detect if link is down and attempt to set up

With this we still get the "network is down" error, but after the 2nd
retry the interface is up and the dhcp request works.

plugins/ipam/dhcp/lease.go

@@ -118,6 +118,13 @@ func (l *DHCPLease) acquire() error {
 	}
 	defer c.Close()
 
+	if (l.link.Attrs().Flags & net.FlagUp) != net.FlagUp {
+		log.Printf("Link %q down. Attempting to set up", l.link.Attrs().Name)
+		if err = netlink.LinkSetUp(l.link); err != nil {
+			return err
+		}
+	}
+
 	pkt, err := backoffRetry(func() (*dhcp4.Packet, error) {
 		ok, ack, err := c.Request()
 		switch {