wgengine/magicsock: remove allocs in UDP write, use new netaddr.PutUDPAddr

The allocs were only introduced yesterday with a TODO. Now they're gone again.

go.mod

@@ -29,6 +29,6 @@ require (
 	golang.org/x/sys v0.0.0-20200501052902-10377860bb8e
 	golang.org/x/time v0.0.0-20191024005414-555d28b269f0
 	honnef.co/go/tools v0.0.1-2020.1.4 // indirect
-	inet.af/netaddr v0.0.0-20200629220211-f44a6d25c536
+	inet.af/netaddr v0.0.0-20200701171350-6509743f79d9
 	rsc.io/goversion v1.2.0
 )

go.sum

@@ -162,5 +162,7 @@ inet.af/netaddr v0.0.0-20200513162223-787f13e36cbe h1:WjJ6wZhXEWQA3FFSwOjG8tO2q1
 inet.af/netaddr v0.0.0-20200513162223-787f13e36cbe/go.mod h1:qqYzz/2whtrbWJvt+DNWQyvekNN4ePQZcg2xc2/Yjww=
 inet.af/netaddr v0.0.0-20200629220211-f44a6d25c536 h1:XFVw2MVOtmHBidx70M+I6vIw2F6f55UyXvkiKfIrE38=
 inet.af/netaddr v0.0.0-20200629220211-f44a6d25c536/go.mod h1:qqYzz/2whtrbWJvt+DNWQyvekNN4ePQZcg2xc2/Yjww=
+inet.af/netaddr v0.0.0-20200701171350-6509743f79d9 h1:F41nQsn8UGDPDXsOPwZQiaK8BmItPzyb0PYgjJSYIzw=
+inet.af/netaddr v0.0.0-20200701171350-6509743f79d9/go.mod h1:qqYzz/2whtrbWJvt+DNWQyvekNN4ePQZcg2xc2/Yjww=
 rsc.io/goversion v1.2.0 h1:SPn+NLTiAG7w30IRK/DKp1BjvpWabYgxlLp/+kx5J8w=
 rsc.io/goversion v1.2.0/go.mod h1:Eih9y/uIBS3ulggl7KNJ09xGSLcuNaLgmvvqa07sgfo=

wgengine/magicsock/magicsock.go

@@ -805,8 +805,9 @@ var errDropDerpPacket = errors.New("too many DERP packets queued; dropping")
 
 // sendUDP sends UDP packet b to ipp.
 func (c *Conn) sendUDP(ipp netaddr.IPPort, b []byte) error {
-	addr := ipp.UDPAddr() // TOOD(bradfitz): add alloc-free netaddr.WriteTo helper
-	return c.sendUDPStd(addr, b)
+	ua := ipp.UDPAddr()
+	defer netaddr.PutUDPAddr(ua)
+	return c.sendUDPStd(ua, b)
 }
 
 func (c *Conn) sendUDPStd(addr *net.UDPAddr, b []byte) (err error) {