bcache: add missing SPDX header

The SPDX header is missing fro closure.c, super.c and util.c, this
patch adds SPDX header for GPL-2.0 into these files.
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/closure.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Asynchronous refcounty things
  *

drivers/md/bcache/super.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * bcache setup/teardown code, and some metadata io - read a superblock and
  * figure out what to do with it.

drivers/md/bcache/util.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * random utiility code, for bcache but in theory not specific to bcache
  *