ext4_blkdev_remove() passes a wrong holder pointer to blkdev_put() which
triggers a warning there. Fix it.

Fixes: 2736e8ee ("block: use the holder as indication for exclusive opens")
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230622165107.13687-1-jack@suse.czSigned-off-by: Jens Axboe <axboe@kernel.dk>