md/bitmap: use sector_div for sector_t divisions

neilb: modified to not corrupt ->resync_max_sectors.

sector_div usage fixed by Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/bitmap.c

@@ -571,9 +571,10 @@ static int bitmap_read_sb(struct bitmap *bitmap)
 re_read:
 	/* If cluster_slot is set, the cluster is setup */
 	if (bitmap->cluster_slot >= 0) {
-		sector_t bm_blocks;
+		sector_t bm_blocks = bitmap->mddev->resync_max_sectors;
 
-		bm_blocks = bitmap->mddev->resync_max_sectors / (bitmap->mddev->bitmap_info.chunksize >> 9);
+		sector_div(bm_blocks,
+			   bitmap->mddev->bitmap_info.chunksize >> 9);
 		bm_blocks = bm_blocks << 3;
 		bm_blocks = DIV_ROUND_UP_SECTOR_T(bm_blocks, 4096);
 		bitmap->mddev->bitmap_info.offset += bitmap->cluster_slot * (bm_blocks << 3);