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

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>