btrfs: use btrfs_bio_for_each_sector in btrfs_check_read_dio_bio

Use the new btrfs_bio_for_each_sector iterator to simplify
btrfs_check_read_dio_bio.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>