internal/fmtsort: makeChans pin pointer

Complete TODO.

For golang#49431

Change-Id: Ifc79741ad44cd9631be49d4674db0ee52bbf6486

Author: qiulaidongfeng

src/internal/fmtsort/sort_test.go

@@ -9,6 +9,7 @@ import (
 	"internal/fmtsort"
 	"math"
 	"reflect"
+	"runtime"
 	"sort"
 	"strings"
 	"testing"
@@ -194,8 +195,11 @@ var (
 
 func makeChans() []chan int {
 	cs := []chan int{make(chan int), make(chan int), make(chan int)}
-	// Order channels by address. See issue #49431.
-	// TODO: pin these pointers once pinning is available (#46787).
+	var pin runtime.Pinner
+	for i := range cs {
+		pin.Pin(&cs[i])
+	}
+	defer pin.Unpin()
 	sort.Slice(cs, func(i, j int) bool {
 		return uintptr(reflect.ValueOf(cs[i]).UnsafePointer()) < uintptr(reflect.ValueOf(cs[j]).UnsafePointer())
 	})