md/raid1: remove unnecessary BUG_ON

Since bitmap_start_sync will not return until
sync_blocks is not less than PAGE_SIZE>>9, so
the BUG_ON is not needed anymore.
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid1.c

@@ -2695,7 +2695,6 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
 			    !conf->fullsync &&
 			    !test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery))
 				break;
-			BUG_ON(sync_blocks < (PAGE_SIZE>>9));
 			if ((len >> 9) > sync_blocks)
 				len = sync_blocks<<9;
 		}