follow the psutil principles, do nothing if can not find root device

disk/disk_linux.go

@@ -349,12 +349,11 @@ func PartitionsWithContext(ctx context.Context, all bool) ([]PartitionStat, erro
 
 			// /dev/root is not the real device name
 			// so we get the real device name from its major/minor number
-			if d.Device == "/dev/root" && d.Fstype != "nfs" {
+			if d.Device == "/dev/root" {
 				devpath, err := os.Readlink(common.HostSys("/dev/block/" + blockDeviceID))
-				if err != nil {
-					return nil, err
+				if err == nil {
+					d.Device = strings.Replace(d.Device, "root", filepath.Base(devpath), 1)
 				}
-				d.Device = strings.Replace(d.Device, "root", filepath.Base(devpath), 1)
 			}
 		}
 		ret = append(ret, d)