wgengine/magicsock: simplify legacy endpoint DstToString

Legacy endpoints (addrSet) currently reconstruct their dst string when requested.

Instead, store the dst string we were given to begin with.
In addition to being simpler and cheaper, this makes less code
aware of how to interpret endpoint strings.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/magicsock/legacy.go

@@ -43,6 +43,7 @@ func (c *Conn) createLegacyEndpointLocked(pk key.Public, addrs string) (conn.End
 		Logf:      c.logf,
 		publicKey: pk,
 		curAddr:   -1,
+		rawdst:    addrs,
 	}
 
 	if addrs != "" {
@@ -384,6 +385,9 @@ type addrSet struct {
 	// set to a better one. This is only to suppress some
 	// redundant logs.
 	loggedLogPriMask uint32
+
+	// rawdst is the destination string from/for wireguard-go.
+	rawdst string
 }
 
 // derpID returns this addrSet's home DERP node, or 0 if none is found.
@@ -426,17 +430,7 @@ func (a *addrSet) DstToBytes() []byte {
 	return packIPPort(a.dst())
 }
 func (a *addrSet) DstToString() string {
-	var addrs []string
-	for _, addr := range a.ipPorts {
-		addrs = append(addrs, addr.String())
-	}
-
-	a.mu.Lock()
-	defer a.mu.Unlock()
-	if a.roamAddr != nil {
-		addrs = append(addrs, a.roamAddr.String())
-	}
-	return strings.Join(addrs, ",")
+	return a.rawdst
 }
 func (a *addrSet) DstIP() net.IP {
 	return a.dst().IP.IPAddr().IP // TODO: add netaddr accessor to cut an alloc here?