Fix S3 multipart upload pagination loop condition

The loop that iterates over paginated lists of S3 multipart upload parts
appears to be using the wrong variable in its loop condition. Nothing
inside the loop affects the value of `resp.IsTruncated`, so this loop
will either be wrongly skipped or loop forever.

It looks like this is a regression caused by commit
7736319f2e. The return value of
`ListMultipartUploads` used to be assigned to a variable named `resp`,
but it was renamed to `partsList` without updating the for loop
condition.

I believe this is causing an error we're seeing with large layer uploads
at commit time:

    upload resumed at wrong offset: 5242880000 != 5815706782

Missing parts of the multipart S3 upload would cause an incorrect size
calculation in `newWriter`.

Signed-off-by: Aaron Lehmann <alehmann@netflix.com>

registry/storage/driver/s3-aws/s3.go

@@ -690,7 +690,7 @@ func (d *driver) Writer(ctx context.Context, path string, appendParam bool) (sto
 				return nil, parseError(path, err)
 			}
 			allParts = append(allParts, partsList.Parts...)
-			for *resp.IsTruncated {
+			for *partsList.IsTruncated {
 				partsList, err = d.S3.ListParts(&s3.ListPartsInput{
 					Bucket:           aws.String(d.Bucket),
 					Key:              aws.String(key),