block: cleanup variable naming in get_max_io_size

get_max_io_size has a very odd choice of variables names and
initialization patterns.  Switch to more descriptive names and more
clear initialization of them.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220614090934.570632-5-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -164,18 +164,16 @@ static struct bio *blk_bio_write_zeroes_split(struct request_queue *q,
 static inline unsigned get_max_io_size(struct request_queue *q,
 				       struct bio *bio)
 {
-	unsigned sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector, 0);
-	unsigned max_sectors = sectors;
 	unsigned pbs = queue_physical_block_size(q) >> SECTOR_SHIFT;
 	unsigned lbs = queue_logical_block_size(q) >> SECTOR_SHIFT;
-	unsigned start_offset = bio->bi_iter.bi_sector & (pbs - 1);
-
-	max_sectors += start_offset;
-	max_sectors &= ~(pbs - 1);
-	if (max_sectors > start_offset)
-		return max_sectors - start_offset;
-
-	return sectors & ~(lbs - 1);
+	unsigned max_sectors, start, end;
+
+	max_sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector, 0);
+	start = bio->bi_iter.bi_sector & (pbs - 1);
+	end = (start + max_sectors) & ~(pbs - 1);
+	if (end > start)
+		return end - start;
+	return max_sectors & ~(lbs - 1);
 }
 
 static inline unsigned get_max_segment_size(const struct request_queue *q,