wgengine/magicsock: use derp-region-as-magic-AddrPort hack in fewer places

And fix up a bogus comment and flesh out some other comments.

Updates #cleanup

Change-Id: Ia60a1c04b0f5e44e8d9587914af819df8e8f442a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -7,6 +7,7 @@ package magicsock
 
 import (
 	"bufio"
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -1154,7 +1155,8 @@ func (c *Conn) sendAddr(addr netip.AddrPort, pubKey key.NodePublic, b []byte) (s
 		return c.sendUDP(addr, b)
 	}
 
-	ch := c.derpWriteChanOfAddr(addr, pubKey)
+	regionID := int(addr.Port())
+	ch := c.derpWriteChanForRegion(regionID, pubKey)
 	if ch == nil {
 		metricSendDERPErrorChan.Add(1)
 		return false, nil
@@ -1165,8 +1167,7 @@ func (c *Conn) sendAddr(addr netip.AddrPort, pubKey key.NodePublic, b []byte) (s
 	// to derpWriteRequest and waited for derphttp.Client.Send to
 	// complete, but that's too slow while holding wireguard-go
 	// internal locks.
-	pkt := make([]byte, len(b))
-	copy(pkt, b)
+	pkt := bytes.Clone(b)
 
 	select {
 	case <-c.donec: