Btrfs-progs: fix to make list specified directory's subvolumes work

Steps to reproduce:
 # mkfs.btrfs -f /dev/sda8
 # mount /dev/sda8 /mnt
 # mkdir /mnt/subvolumes
 # btrfs sub create /mnt/subvolumes/subv1
 # btrfs sub create /mnt/subvolumes/subv1/subv1.1
 # btrfs sub list -o /mnt/subvolumes/subv1   <----we did not list anything

The problem is that we don't set @top_id right, fix it.

Reported-by: Alex <alex@bpmit.com>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

btrfs-list.c

@@ -578,23 +578,18 @@ static int resolve_root(struct root_lookup *rl, struct root_info *ri,
  full_path = strdup(found->path);
  len = add_len;
  }
+ if (!ri->top_id)
+ ri->top_id = found->ref_tree;
 
  next = found->ref_tree;
-
- if (next == top_id) {
- ri->top_id = top_id;
+ if (next == top_id)
  break;
- }
-
  /*
  * if the ref_tree = BTRFS_FS_TREE_OBJECTID,
  * we are at the top
  */
- if (next == BTRFS_FS_TREE_OBJECTID) {
- ri->top_id = next;
+ if (next == BTRFS_FS_TREE_OBJECTID)
  break;
- }
-
  /*
  * if the ref_tree wasn't in our tree of roots, the
  * subvolume was deleted.