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

include/linux/blkdev.h

@@ -815,7 +815,8 @@ static inline bool rq_mergeable(struct request *rq)
 
 static inline bool blk_write_same_mergeable(struct bio *a, struct bio *b)
 {
-	if (bio_data(a) == bio_data(b))
+	if (bio_page(a) == bio_page(b) &&
+	    bio_offset(a) == bio_offset(b))
 		return true;
 
 	return false;