Btrfs: Avoid 64 bit div for RAID10

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 0e658c1..c11b6fd 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -724,7 +724,7 @@
 	if (type & (BTRFS_BLOCK_GROUP_RAID1 | BTRFS_BLOCK_GROUP_DUP))
 		*num_bytes = calc_size;
 	else if (type & BTRFS_BLOCK_GROUP_RAID10)
-		*num_bytes = calc_size * num_stripes / sub_stripes;
+		*num_bytes = calc_size * (num_stripes / sub_stripes);
 	else
 		*num_bytes = calc_size * num_stripes;