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 <[email protected]>
Reviewed-by: Shenghui Wang <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
3 files changed