Btrfs: remove unnecessary code of stripe_index assignment in __btrfs_map_block

stripe_index's value was set again in latter line:
stripe_index = 0;
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>

fs/btrfs/volumes.c

@@ -5166,9 +5166,7 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info, int rw,
 
 			/* push stripe_nr back to the start of the full stripe */
 			stripe_nr = raid56_full_stripe_start;
-			do_div(stripe_nr, stripe_len);
-
-			stripe_index = do_div(stripe_nr, nr_data_stripes(map));
+			do_div(stripe_nr, stripe_len * nr_data_stripes(map));
 
 			/* RAID[56] write or recovery. Return all stripes */
 			num_stripes = map->num_stripes;