fix: add more optimizations to putMetacacheObject() (#12916)

- avoid extra lookup for 'xl.meta' since we are
  definitely sure that it doesn't exist.

- use this in newMultipartUpload() as well

- also additionally do not write with O_DSYNC
  to avoid loading the drives, instead create
  'xl.meta' for listing operations without
  O_DSYNC since these are ephemeral objects.

- do the same with newMultipartUpload() since
  it gets synced when the PutObjectPart() is
  attempted, we do not need to tax newMultipartUpload()
  instead.

cmd/erasure-multipart.go

@@ -347,8 +347,9 @@ func (er erasureObjects) newMultipartUpload(ctx context.Context, bucket string,
 	// Fill all the necessary metadata.
 	// Update `xl.meta` content on each disks.
 	for index := range partsMetadata {
-		partsMetadata[index].Metadata = opts.UserDefined
+		partsMetadata[index].Fresh = true
 		partsMetadata[index].ModTime = modTime
+		partsMetadata[index].Metadata = opts.UserDefined
 	}
 
 	uploadID := mustGetUUID()