core/mount: add benchmark test for GetUsernsFD

```bash
$ go test -bench=.  -benchmem ./ -exec sudo
goos: linux
goarch: amd64
pkg: github.com/containerd/containerd/v2/core/mount
cpu: AMD Ryzen 7 5800H with Radeon Graphics
BenchmarkBatchRunGetUsernsFD_Concurrent1-16                 2398            532424 ns/op            1145 B/op         43 allocs/op
BenchmarkBatchRunGetUsernsFD_Concurrent10-16                 343           3701695 ns/op           11552 B/op        421 allocs/op
PASS
ok      github.com/containerd/containerd/v2/core/mount  2.978s
```

Signed-off-by: Wei Fu <fuweid89@gmail.com>

core/mount/mount_idmapped_linux_test.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"sync"
 	"syscall"
 	"testing"
 
@@ -29,6 +30,34 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+func BenchmarkBatchRunGetUsernsFD_Concurrent1(b *testing.B) {
+	for range b.N {
+		benchmarkBatchRunGetUsernsFD(1)
+	}
+}
+
+func BenchmarkBatchRunGetUsernsFD_Concurrent10(b *testing.B) {
+	for range b.N {
+		benchmarkBatchRunGetUsernsFD(10)
+	}
+}
+
+func benchmarkBatchRunGetUsernsFD(n int) {
+	var wg sync.WaitGroup
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		go func() {
+			defer wg.Done()
+			fd, err := getUsernsFD(testUIDMaps, testGIDMaps)
+			if err != nil {
+				panic(err)
+			}
+			fd.Close()
+		}()
+	}
+	wg.Wait()
+}
+
 var (
 	testUIDMaps = []syscall.SysProcIDMap{
 		{ContainerID: 1000, HostID: 0, Size: 100},