zram: cleanup zram_remove

Remove the bdev variable and just use the gendisk pointed to by the
zram_device directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220330052917.2566582-4-hch@lst.de


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