loop: Remove the unnecessary bdev checks and unused bdev variable

The lo->lo_device can't be null if the lo->lo_backing_file is set.
So let's remove the unnecessary bdev checks and the entire bdev
variable in __loop_clr_fd() since the lo->lo_backing_file is already
checked before.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210922123711.187-4-xieyongji@bytedance.com


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