bcache: move closure debug file into debug directory

In current code closure debug file is outside of debug directory
and when unloading module there is lack of removing operation
for closure debug file, so it will cause creating error when trying
to reload  module.

This patch move closure debug file into "bcache" debug direcory
so that the file can get deleted properly.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Reviewed-by: Tang Junhui <tang.junhui@zte.com.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>