tun: use x/sys/unix IoctlIfreq/NewIfreq to set MTU on Linux

The manual struct packing was suspect:
https://github.com/tailscale/tailscale/issues/11899

And no need for doing it manually if there's API for it already.

Updates tailscale/tailscale#11899

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

tun/tun_linux.go

@@ -269,21 +269,15 @@ func (tun *NativeTun) setMTU(n int) error {
 
 	defer unix.Close(fd)
 
-	// do ioctl call
+	req, err := unix.NewIfreq(name)
-	var ifr [ifReqSize]byte
+	if err != nil {
-	copy(ifr[:], name)
+		return fmt.Errorf("unix.NewIfreq(%q): %w", name, err)
-	*(*uint32)(unsafe.Pointer(&ifr[unix.IFNAMSIZ])) = uint32(n)
+	}
-	_, _, errno := unix.Syscall(
+	req.SetUint32(uint32(n))
-		unix.SYS_IOCTL,
+	err = unix.IoctlIfreq(fd, unix.SIOCSIFMTU, req)
-		uintptr(fd),
+	if err != nil {
-		uintptr(unix.SIOCSIFMTU),
+		return fmt.Errorf("failed to set MTU of TUN device %q: %w", name, err)
-		uintptr(unsafe.Pointer(&ifr[0])),
-	)
-
-	if errno != 0 {
-		return fmt.Errorf("failed to set MTU of TUN device: %w", errno)
 	}
-
 	return nil
 }