xl: check for correct err variable when prepareFile fails in PutObjectPart (#6573)

in xl.PutObjectPart call, prepareFile detected an error when the storage
is exhausted but we were returning the wrong error.

With this commit, users can see the correct error message when their disks
become full.

cmd/xl-v1-multipart.go

@@ -343,7 +343,7 @@ func (xl xlObjects) PutObjectPart(ctx context.Context, bucket, object, uploadID
 	// Delete the temporary object part. If PutObjectPart succeeds there would be nothing to delete.
 	defer xl.deleteObject(ctx, minioMetaTmpBucket, tmpPart, writeQuorum, false)
 	if data.Size() > 0 || data.Size() == -1 {
-		if pErr := xl.prepareFile(ctx, minioMetaTmpBucket, tmpPartPath, data.Size(), onlineDisks, xlMeta.Erasure.BlockSize, xlMeta.Erasure.DataBlocks, writeQuorum); err != nil {
+		if pErr := xl.prepareFile(ctx, minioMetaTmpBucket, tmpPartPath, data.Size(), onlineDisks, xlMeta.Erasure.BlockSize, xlMeta.Erasure.DataBlocks, writeQuorum); pErr != nil {
 			return pi, toObjectErr(pErr, bucket, object)
 
 		}