dm zoned: add SPDX license identifiers

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-zoned-metadata.c

+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright (C) 2017 Western Digital Corporation or its affiliates.
  *

drivers/md/dm-zoned-reclaim.c

+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright (C) 2017 Western Digital Corporation or its affiliates.
  *

drivers/md/dm-zoned-target.c

+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright (C) 2017 Western Digital Corporation or its affiliates.
  *

drivers/md/dm-zoned.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (C) 2017 Western Digital Corporation or its affiliates.
  *