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.dk


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