Fix shrink partitions race condition. (#26)

After changes to a partition table has been made, including resizing a
partition, all the disk's partition nodes under /dev are removed and
then recreated. We need to ensure we wait for this to complete before
trying to shrink the next partition.

Also, remove the call to `partprobe` since calls to `parted` do this
automatically. So, the `partprobe` call is redundant.

toolkit/tools/pkg/imagecustomizerlib/shrinkfilesystems.go

@@ -107,10 +107,11 @@ func shrinkFilesystems(imageLoopDevice string, verity []imagecustomizerapi.Verit
 			return fmt.Errorf("failed to resizepart %s with parted (and flock):\n%v", partitionLoopDevice, stderr)
 		}
 
-		// Re-read the partition table
-		err = refreshPartitions(imageLoopDevice)
+		// Changes to the partition table causes all of the disk's parition /dev nodes to be deleted and then
+		// recreated. So, wait for that to finish.
+		err = diskutils.WaitForDevicesToSettle()
 		if err != nil {
-			return err
+			return fmt.Errorf("failed to list disk (%s) partitions:\n%w", partitionLoopDevice, err)
 		}
 	}
 	return nil