block: add a bdev_nr_bytes helper

Add a helper to query the size of a block device in bytes.  This
will be used to remove open coded access to ->bd_inode.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211018101130.1838532-3-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/genhd.h

@@ -236,9 +236,14 @@ static inline sector_t get_start_sect(struct block_device *bdev)
 	return bdev->bd_start_sect;
 }
 
+static inline loff_t bdev_nr_bytes(struct block_device *bdev)
+{
+	return i_size_read(bdev->bd_inode);
+}
+
 static inline sector_t bdev_nr_sectors(struct block_device *bdev)
 {
-	return i_size_read(bdev->bd_inode) >> 9;
+	return bdev_nr_bytes(bdev) >> SECTOR_SHIFT;
 }
 
 static inline sector_t get_capacity(struct gendisk *disk)