Close tarsplit gzip writer when creating tar-split.json.gz files during layer migration

There is a missing call to Close on the gzip.Writer that is used to
compress newly created tar-split files during layer migration. This can
result in corrupt tar-split files that later cause docker push and
docker save to fail. The Close call is necessary to flush buffered data
to the stream.

Fixes: #20104

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>