Make sure to update modTime in erasure metadata (#6296)

This is to ensure that when we update xl.json with new
parts have the latest modtime, in-turn avoids consistency
issues when the disk is offline.

cmd/xl-v1-multipart.go

@@ -462,6 +462,7 @@ func (xl xlObjects) PutObjectPart(ctx context.Context, bucket, object, uploadID
 		if disk == OfflineDisk {
 			continue
 		}
+		partsMetadata[i].Stat = xlMeta.Stat
 		partsMetadata[i].Parts = xlMeta.Parts
 		partsMetadata[i].Erasure.AddChecksumInfo(ChecksumInfo{partSuffix, DefaultBitrotAlgorithm, writers[i].Sum()})
 	}