fs/proc: report PID-s from a pid namespace of the proc mount

Right now, we can find more than one process with the 1 PID in /proc.

$ for i in `seq 10`; do
> unshare -fp sleep 1000 &
> done

$ ls /proc
1  1  1  1  12  18  24  29  6            loadavg  net   sys          version
1  1  1  1  16  20  26  32  cpuinfo      meminfo  self  thread-self
1  1  1  1  17  21  28  36  filesystems  mounts   stat  uptime

PiperOrigin-RevId: 272506593

pkg/sentry/fs/proc/proc.go

@@ -230,7 +230,7 @@ func (rpf *rootProcFile) Readdir(ctx context.Context, file *fs.File, ser fs.Dent
 	// But for whatever crazy reason, you can still walk to the given node.
 	for _, tg := range rpf.iops.pidns.ThreadGroups() {
 		if leader := tg.Leader(); leader != nil {
-			name := strconv.FormatUint(uint64(tg.ID()), 10)
+			name := strconv.FormatUint(uint64(rpf.iops.pidns.IDOfThreadGroup(tg)), 10)
 			m[name] = fs.GenericDentAttr(fs.SpecialDirectory, device.ProcDevice)
 			names = append(names, name)
 		}