block: switch all files cleared marked as GPLv2 to SPDX tags

All these files have some form of the usual GPLv2 boilerplate.  Switch
them to use SPDX tags instead.

Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-flush.c b/block/blk-flush.c
index d95f948..aedd932 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -1,11 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Functions to sequence PREFLUSH and FUA writes.
  *
  * Copyright (C) 2011		Max Planck Institute for Gravitational Physics
  * Copyright (C) 2011		Tejun Heo <tj@kernel.org>
  *
- * This file is released under the GPLv2.
- *
  * REQ_{PREFLUSH|FUA} requests are decomposed to sequences consisted of three
  * optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
  * properties and hardware capability.