btrfs-progs: inspect tree-stats: print number of nodes for each level
    
    The node/leaf stats have been calculated but never displayed. Moreover,
    a more detailed information about counts on each level can be useful,
    add it to the output of tree-stats.
    
    Example output:
    
            Levels: 3
            Total nodes: 25692
                    On level 0: 25601
                    On level 1: 90
                    On level 2: 1
    
    Issue: #266
    Signed-off-by: David Sterba <dsterba@suse.com>