mtd: blktrans: do blk_cleanup_queue when it is really safe to do so

I was calling it in del_mtd_blktrans_dev, but ->request_fn could
still be running at that point, thus defer this call
to blktrans_dev_release

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>