dm writecache: commit just one block, not a full page

Some architectures have pages larger than 4k and committing a full
page causes needless overhead.

Fix this by writing a single block when committing the superblock.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -532,11 +532,7 @@ static void ssd_commit_superblock(struct dm_writecache *wc)
 
 	region.bdev = wc->ssd_dev->bdev;
 	region.sector = 0;
-	region.count = PAGE_SIZE >> SECTOR_SHIFT;
-
-	if (unlikely(region.sector + region.count > wc->metadata_sectors))
-		region.count = wc->metadata_sectors - region.sector;
-
+	region.count = wc->block_size >> SECTOR_SHIFT;
 	region.sector += wc->start_sector;
 
 	req.bi_op = REQ_OP_WRITE;