fs: bdev: fix conflicting comment from lookup_bdev

fs: bdev: fix conflicting comment from lookup_bdev

We switched to directly use dev_t to get block device, lookup changed the
meaning of use, now we fix this conflicting comment.

Fixes: f87430626b2d ("block: remove i_bdev")
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211021071344.1600362-1-liu.yun@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>