block: use new bdev_nr_bytes() helper for blkdev_{read,write}_iter()

We have new helpers for this, use them rather than the slower inode
size reads. This makes the read/write path consistent with most of
the rest of block as well.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/a72767cd-3c6d-47f7-80f4-aa025a17b2cb@kernel.dkSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/fops.c

@@ -460,7 +460,7 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 {
 	struct block_device *bdev = iocb->ki_filp->private_data;
 	struct inode *bd_inode = bdev->bd_inode;
-	loff_t size = i_size_read(bd_inode);
+	loff_t size = bdev_nr_bytes(bdev);
 	struct blk_plug plug;
 	size_t shorted = 0;
 	ssize_t ret;
@@ -498,7 +498,7 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 static ssize_t blkdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 {
 	struct block_device *bdev = iocb->ki_filp->private_data;
-	loff_t size = i_size_read(bdev->bd_inode);
+	loff_t size = bdev_nr_bytes(bdev);
 	loff_t pos = iocb->ki_pos;
 	size_t shorted = 0;
 	ssize_t ret;