block: stop using bio_data() in blk_write_same_mergeable

While the Write Same page currently always is in low-level it is just
as easy and safer to just compare the page and offset directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>