block: use bdev_nr_bytes instead of open coding it in blkdev_fallocate

Use the proper helper to read the block device size.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20211018101130.1838532-25-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/fops.c

@@ -535,7 +535,7 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
 		return -EOPNOTSUPP;
 
 	/* Don't go off the end of the device. */
-	isize = i_size_read(bdev->bd_inode);
+	isize = bdev_nr_bytes(bdev);
 	if (start >= isize)
 		return -EINVAL;
 	if (end >= isize) {