loop: reorder loop_exit

Unregister the misc and blockdevice first to prevent further access,
and only then iterate to remove the devices.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Link: https://lore.kernel.org/r/20210623145908.92973-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>