md: use sector_t in bitmap_get_counter

bitmap_get_counter returns the number of sectors covered
by the counter in a pass-by-reference variable.
In some cases this can be very large, so make it a sector_t
for safety.

Signed-off-by: NeilBrown <neilb@suse.de>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 31140d1..8abc159 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -4360,7 +4360,7 @@
 	raid5_conf_t *conf = mddev->private;
 	struct stripe_head *sh;
 	sector_t max_sector = mddev->dev_sectors;
-	int sync_blocks;
+	sector_t sync_blocks;
 	int still_degraded = 0;
 	int i;