The toplevel subvolume is special and the other listing code leaves it
out so we have to add several special cases to handle it. There's no
backreference so the path is built artificially. New helper
btrfs_get_toplevel_subvol is a reduced version of btrfs_get_subvol.

There's some information usually missing for the toplevel subvolume, eg.
the uuid or creation info. This has to be fixed on the mkfs side, the
other subvolumes are created by kernel.

Example:
/mnt
        Name:                   <FS_TREE>
        UUID:                   -
        Parent UUID:            -
        Received UUID:          -
        Creation time:          -
        Subvolume ID:           5
        Generation:             233
        Gen at creation:        0
        Parent ID:              0
        Top level ID:           0
        Flags:                  -
        Snapshot(s):
                                subv1

Signed-off-by: David Sterba <dsterba@suse.com>