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/raid10.c b/drivers/md/raid10.c
index f0d082f..387fe4b 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1820,7 +1820,7 @@
 	int disk;
 	int i;
 	int max_sync;
-	int sync_blocks;
+	sector_t sync_blocks;
 
 	sector_t sectors_skipped = 0;
 	int chunks_skipped = 0;